it depends on many variables- how many prints you're doing, what you coat the screen with, how you store it, how well you clean it, if you let emulsion get into the frame (it eats away at the screen)...
could you be a little more specific? what are your expectations for what you'll be doing?
Don't forget about the accidental sharp object to the screen. It is amazing the wear and tear that a screen goes through and nothing happens, then in that one split second a pair of scissors touches the screen and rip....
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n o r m www.normsbrand.com - Saving the World, One Chest at a Time
like i said, there are many variables, but assuming you did everything correctly , and avoided sharp objects- maybe a year or more. i have some screens that are a year old and they work just fine, but we're always extremely careful with storing them and cleaning them so they don't clog.
first you have to degrease the screen with a printing degreaser- or baking soda and water. the abrasiveness will help hold the emulsion to the screen- which is really important if you're going to store it for a while.
then tape the screen- you can use printing tape, but i use masking tape because it's cheaper and easier to use (make sure you take it off once you are done printing, then retape before another print!). tape so no emulsion or ink can get in between the screen and the frame. tape both sides of the screen- it should take up no more than inch on each side.
theeeeen... carefully coat the screen- make sure you dont get any fish eye drip marks...those are bad. BAD!
after you are done printing, make sure you really clean the ink off- there will be some residue, but that's ok. like i said, remove the tape.
store it somewhere cool and away from sunlight- prop it up against a wall- maybe cover it with a cloth so it doesn't get poked by something sharp.
i forgot to add that you need a special ink cleaner for plastisol inks- wear rubber gloves and a mask (or go outside) because they smell awful- at least ours does. i always fill a bag with water, then put the oily, solvent ladden rags in there and squish out the air.
can't be too careful when your house burned down from oily rags (not from me! i was a month old!) once before...
How long would a screen last me? 
