How often should rear suspension frame linkages be serviced?
No, it's not typical maintenance, but you do have to take it apart to either replace or perhaps just service (degrease and relubricate) the bearings once in a while. Which you do may depend on what you found and how much disassembly of the bike's other systems it took to get there, which has become everything but the fork and front brake on some bikes, where the link and the internal hose/housing/wire routing are tightly intertwined. You do this more for bearing wear than creaks per se in most cases, although a creak may be the symptom that gets you looking.

Riding conditions have a major influence on the level of maintenance required. I live in the US Pacific Northwest, and we have a lot of fine clay soil and moisture that at times seems to create an almost perfect assault on suspension bearings, which would have a much easier life in other regions. 

The more common thing to do for creaks is drop light oil or dry lube into the pivots, cleaning as well as possible first to avoid working in grit. Also make sure to keep all the hardware properly torqued. Torquing and lubing is the first thing to check when there's a creak from a linkage pivot.