That's a great question!
The time before your voice mail kicks in - i.e., the number of rings before it switches over to voice mail - is set by the parent carrier network (Sprint, in the case of TextNow).
Currently, there's no way to set the number of rings on your end, and the time before going to voice mail can vary pretty significantly - I've seen it set anywhere between three rings and eleven rings.
Our engineers are in the process to see if there any way our customers can set the number of rings, so we'll hopefully have that included in a future update.
Thanks for your support of TextNow!