Can be anywhere from 6 -13 months. Best gauge of it is to find out when their mama had her first heat. I have one that is first heat at 11 mo. and one that is in first heat at 7 1/2 - so it can really vary. General rule is next season is 6 months (from start of previous one) - but it can sometimes be a little shorter or longer These little guys like to keep us guessing :)
Well Sue said it nicely and I dont think there's much to add really! Good job girlie! All the dogs I've had in the past have come into season at 6 months, but more often than not Chihuahauas come into season for the first time later than that. Ellie's first was at 9 months, and her spaces between are about 7 months. I imagine the reason smaller breeds develop slower is because they live longer so aren't in such a hurry to reproduce.