I haven't used Infants Friend but one of the girls from Mother's Group (actually two of them) have and one said it worked within about 2 hours (her baby was constipated). I imagine that like all things ingested, the time it takes to work is going to depend a lot on your babies metabolism and digestive system.
If you think you might have trouble getting bubs to take it, try giving it before a feed because then the feed will wash it down (so to speak) and also get rid of the taste for them.

HTH

MG