Well they start out with the backhand spring. When I took tumbling classes my instructor told me it could take me 1 month to learn, or years! I got my backhand spring 2 years after my tumbling classes, so it varies for different people. If you aren't afraid to throw it, you'll get it in no time. As for running tumbling, it could take some years.

i started tumbling classes and i didnt even know how to do a cart wheel. its now less than a year later and im really close to having my back handspring. you just have to not be afraid and youll get it pretty quick. they do wonders :-) good luck