How to calculate the reset time?
The best way, if possible, is to tune the parameters rather than 'calculating' them. 

The reset time, or integrating time constant, \$T_i\$, is roughly the time your controller takes to overcome steady-state errors. 

To tune it, you can just start from a high value and slowly reduce it checking out the performance of a step-response (quick change in reference to your controller). As soon as you are satisfied with the promptness of the response and the (eventual) amount of overshoot you introduced, you are finished.

If you really want to 'calculate' the value, you just have to choose one of the hundreds avaliable PID tuning rules of thumb. Note though that not all of them define \$T_i\$ directly just as a function of the other values, but rather define all parameters together.