Depends on what type of shocks they are, and how heavily used they are. Basically shocks come in three flavours: elastomers(=rubber cushions), oil/spring and oil/air.
Elastomers may wear out, even fracture due to use and age. But if you're staying(almost) exclusively on roads, a collapsed fork may not be an entirely bad thing.
Oil/spring are very robust. They "should" have the oil replaced at some sort of interval, and may eventually wear the seals down to the point of poor shock absorption. But for casual riding with little contamination their lifelength is indefinite, and the advantage of preventative maintenance fairly marginal. Prices being as they are over here, I reckon 4-5 services would equal the price of a new fork if you stick to the inexpensive models.
Oil/air are the most delicate of the lot, with recommended service intervals as low as 30 hours for some models. But that's mainly driven by exposure to contamination, and the amount of compression they regularly see.
For casual riding, check the sag/air pressure every now and then and you're OK.