Depends on what you find acceptable. I've had 2 year old sheets for occasionally trying out a new blade and they work fine...but:
The first thing to degrade is grip. Rubber oxidizes, looses sheen, and generally start becoming noticeably less grippy within 2 months. The degradation is gradual, and you won't notice until you compare directly to a new sheet. Still playable for at least 6-7 months after which looping becomes less effective. Ofcourse this is rubber dependent, I find ESN rubbers loose initial grip and sheen faster than the highly vulcanized chinese and TSP pips, but they tapper off slower. Whereas TSP and 802-40 style pips fail more suddenly (start turning white around the base).
Bounciess degrades much slower. A new sheet of Victas 102 is not notceably faster than one I bought 5 months ago (although much grippier).
I test by rubbing a new ball across the pips at moderate pressure, if it slips too much, I toss it.
For me, ESN pips start breaking (rips at the base) at the sweetspot about 6 months. TSP Spinpips last a bit longer. I've rarely seen an 802-40 rip.
If you want a guess estimate, I usually replace around 4-5 months.
Disclaimer: I like spin-oriented attacking pips. I hit very hard. If you are looking for the disturbing, spin-insensitive type pips, I would say the sponge will hold up at least a year. After which you can replace the sponge only, the low-spin topsheet only gets better with age.