How long can one take 1mg daily of klonopin (clonazepam) before it becomes habit forming? My doctor wrote me a one-time prescription to use as needed until the antidepressant I just started kicks in.

Klonopin (clonazepam) I would follow the doctor's recommendation & not refill the prescription, using Klonopin (clonazepam) only as needed.
Depends . On the person. Once a day Klonopin shouldn't be a problem for several weeks while trying to get established on lexapro (escitalopram). Problems are more likely with benzodiazepines with shorter half life and more frequent doses. (like xanax). Just remember, less is more. Take only as prescribed and for the shortest time possible.
Not long . With regular use, Klonopin (clonazepam) can reach a steady state in your system, days to weeks after which one can develop a physiologic dependence. The good news is that since your anti depressant has become active your can now in initiate a taper to get off the klonopin (clonazepam). This should be done under your physicians supervision. The idea is that now that the antidepressant is working, it's addressing your sympt.
Habit. Clonazepam is a tranquilizer so it is best to only have the one script so that you don't get hooked on the medication.