What's up with the forum s/w logging me out periodically, or when I switch from one topic to another?

I looked to see whether I could find a toggle for this in my account settings, but could not.

What is the time limit? Sometimes I squeeze in a bit of reading and a reply between sixteen other things...and get logged out during pauses to answer the phone/door, manage other processes, etc. I tried finding answers on the forum but had no luck.

thanks