What is the wait time before activating a tourist visa obtained in Laos for entry into Thailand, and how do extra days in Laos affect the visa duration?

A single entry tourist visa is valid for 90 days. On entering you'll get stamped in for 60 days.

As for waiting before entering Thailand there should be no reason / benefit why you don't go straight to the border and enter Thailand.

Of course if you have an extensive history of back to back SETV / visa exempt you could be "interviewed" by the immigration officer. Although very unlikely at either the Nong khai or mukdahan crossing.

I think its a few months until it expires.. You get granted the visa whatever day.. but the clock doesn't start until you cross the border into Thailand.

If you look at the image on this page 'Thai Visa Advice' there is an "Enter Before" date. I think its six month - so with SETV you have a few months.