However confident you are about your own good health, remember that you may be able to use sick time to care for a loved one while he or she is sick. If you have old parents, or have or plan to have a kid, you may need that time.

a couple weeks worth of accrued sick time is a pretty decent amount to have as a safety net. and i wouldn't worry too much about taking off one day as "sick"/mental health day.

but, if you want to take more than that, why not just schedule it in advance with your employer and not worry about whether or not it's going to come back to bite you in the ass?

Don't listen O my Captain. Like Kip Waddle said sick time is like any other benefit. It is there for you to use. You should be responsible in its use but you should definitely use it. I have always used sick time for the occasional "mental health day" and have so far it hasn't impacted my career trajectory, earning potential, or caused the sky to fall in.

As long as you work for a reasonable company and not for Nazis, a customer service call center, or other fascist like organization you should be fine taking the occasional day off and using your sick time.

Do you have long term disability benefits, and if so, how long before it kicks in if you were to get injured? Mine starts after 60 days, and my goal right now is to have a month of combined sick and vacation time accumulated and a months worth of savings in case something happens. A co-worker of mine had a really bad back injury from work, and L&I didn't start making payments until she'd come back to work 2 months later. She had very limited sick/vacation time, and things were pretty stressful for her for a while.

My policy used to always be that if I hadn't used a sick day in 5 months or so, I'd take a mental health day. Which was a pretty good guarantee that I'd get the flu a week later.

Many modern employers have gone to a "paid time off" policy, not specific "sick leave." Your PTO is your own, to be used at your discretion, so long as you don't leave others to pick up your slack. (Of course, many have also used that to combine vacation and sick leave into half as many days off overall, but I digress.)

You know your employer's policies better than anyone here does. If you work for one of the old, hard-line companies, then it's necessary to be careful about using your sick time for personal days off. If you work for a more reasonable company, then just make sure your unscheduled days off aren't creating an unreasonable burden on others.

And there's no way of telling how much you should keep "in the bank" for future issues. If you're never sick, great, but you could still get in a car wreck, develop a chronic illness, etc. To be really "safe," basically, you have to keep as much as your employer allows you to save.

I think it's a poorly-held secret that people take mental health days even though many companies don't sanction them.

If you start using sick leave, be careful about choosing frequent Mondays and Fridays. It's kind of a dead giveaway. Keep a week to ten days in bank.

Also, if you do take a mental health day, get out of town or stay at home. I know of a case where someone was busted for calling in sick on a payday, and was seen later that day shopping away at Northgate Mall - bags a-swinging. She was a troublesome employee - calling in sick a lot (on Fridays) and disappearing at work - and the organization was just looking for an excuse to dismiss her. Still, even if you are the best employee in the world - Employee of the Year - and deserve some time away, when you call in sick and are later seen at a baseball game, your credibility at work will suffer. So will your future with the company.

Are you healthy?

Take it while you can: this is a layoff economy still, and the numbers aren't getting much better. The fact you even still HAVE sick time is amazing to some folks.

One does not, by a long stretch, have to be sick-in-bed in order to take sick leave. Study your employer's handbook: there'll be stipulation there about when you do or don't need a "note".

One of my former employers always accepted 'staying home for a sick child, spouse or family member' as a legit sick day. (Can't tell you how many times my spouse and/or cat was sick).

Perhaps your back 'hurts' (no telling how MUCh it hurts) and 2 days in the spa are just the treatment your kindhearted healthcareadvocate/Reverend ordered...
...just saying.

Following up what the others have said, take O My Captain's advice if you have any reason to suspect that you have a target on your back; otherwise, take Kip and Dan's advice but be sure you don't make a habit of it - you'll fall into some recognizable pattern and that's not good.

That said, you're actual question was how much to leave for a real illness? At least five days' worth. That should cover a bout of the flu or a trip to the hospital.

Do you have a "use it or lose it" policy at your work? If so, try to take those mental health days well before the deadline, or you'll be using them when everyone else is.

Planning on having children? "Care of Newborn" leave (formerly known as maternal leave) is usually paid out of your sick time. Save it all in that case. You'll need it.

Never use sick days for anything other than illness.
Others can tell if you're faking. And if the employer ever asks you for confirmation, you'd better be able to produce: doctor's appt, Rx, drug receipts, etc. SOMETHING to confirm that you were ill.

Also, word spreads among co-workers when you take "mental health days". You get a reputation for taking time off, and then you get stabbed in the back when it comes time for promotions, etc.

Don't do it.