How long can you keep your PSP in sleep mode?

How long can you keep your PSP in that sleep mode where you click the power button up just for a second (not long enough to shut down)?

I do this when I am in the middle of a game and want to continue it the next day - is that a problem?

Thanks for any info. 

I do it all the time on sleep mode.because i get back into the game quicker.

I had problems doing it.

until the battery dies... 

since sleep mode drains incredibly little from the battery, probably something like a year or two.