Hi,

In mixed decay the current is allowed to decay in the fast decay mode upto a certain time then is switched to slow decay mode.

Can anyone tell me if there is a formula to ascertain this time?

 

It will be based on motor application. increased shorting in slow decay mode can limit motor top speed, so it might be used if max. Speed is not a deciding factor.
See if this can increase your understanding...
http://www.piclist.com/techref/io/stepper/decay.htm