are we talking about open limp or limp behind? If the question is when to open limp i'd say its in one and only in one case in poker, and that is NEVER. Seriously guys, open limp is probably the worse habit that player can incorporate in his game, it can generate so many troubles in a hand both preflop and postflop, whether youre having a premium or questionable hand youre either losing value of your premium cuz youre missed one good round of betting, pot is smaller on later streets, harder to get all the money in if you hit the nuts, not to mention that thinking player would instantly have you on a polarised range of either uber premium or utter crap, when i see a limper i tag him as a fish instantly, i take a note of what he is doing when he is isolated how often he limp raise how often he limp fold (i also added stat to my hud with it) so whatever trick that player thinks that he can do with limping is either of a short life or dedicated to be pulled at 2nl and do we want to stay at 2nl for life or want to move up stakes, if second, please delete limp from your vocabulary, its for your own good...
I think it has its place like everything in poker.
If you end up with a bunch of passive players that all want to limp, I will limp with hands that usually get folded to a raise... ie. Suited A-rag, 22, 33, 44, 55, 66, 77 and so on. I would only limp with hands that if I hit can turn into monsters like a set or nut flush, just about everything else goes in the muck. Limping too much is obviously a hole and often can lead to winning small pots and losing big ones.