My vet mentioned that they should (roughly) be 1lb in weight for each month in age so a 6 month old should be 6lb... but it does depend on breed etc - it should give you a rough guide tho. Mine are tiny for their age are 2 months old and only 700grms so waaay off their weight - but think its cos they are twins so were born small (almost same size as the little runt of the litter...)!