Is it a sought after feature do you know or is the chances of it becoming available slim to none?
I've no idea if there are plans to add timeout options to the stock lock screen or not. I seem to recall reading that users experienced the same issue on the S5 but don't know if it was ever addressed. (I'm guessing not.)
You should be able to press and hold the home button when the phone is locked, which will hopefully prevent it from timing out on you during an unlock. The only downside is that too a long press might launch the Google app's search as per the default setting. Other than hunting down a replacement lock screen which supports the S6's fingerprint scanner and allows you change the lock screen timeout (
if one even exists, I haven't bothered checking), I'm unaware of another workaround.
If anyone else has devised a functional solution, hopefully they'll share it.
Edited to add:
C Locker claims to support S Series fingerprint security, but I haven't tested it yet. I might give it a try myself.
Edit #2: Tried the replacement app, didn't like it. It has good intentions and appears decently customizable (more so with widgets and whatnot, at least in the paid version), but it still seems a bit buggy to me. Fingerprint recognition was not as reliable as the stock launcher, although it does support a PIN backup, and it does allow alternative settings for lock screen timeout. I just didn't feel like all the kinks were worked out yet. Uninstalled.