You'd be safe with a .9-1.0 amp charge rate.

You can use your charger to charge your starter box battery but it will take too long. I would recommend you get an overnight wall charger from an automotive store for this. They're inexpensive and work fine. Also down the road you may want to consider a lipo pack for your starter box and your charger will charge it faster. I went that route last year and never looked back. Also a hell of a lot lighter!

There are other factors to consider (motor type, esc, how hard and long you run) but generally your 2s lipo should take about 30-40 mins to charge at 1C.