How long since the last review of a provider should we assume that she is no longer in the business?

I recall seeing someone who hadn't had a new review in over six months.

If their site is active, drop them an email.  If you get no response after three days or so, then that is a better tip off.

Ditto with calling them if they list a number.  If the line is disconnected, consider them retired.

As long as the web site link works, send an e-mail to see if she can see you.  If no response after a few days, move on.

call her and ask

I think it's safe to say that, if you can find a provider's ad anywhere, in any medium, and if it's current, she's still in the biz.  The infrequency of reviews wouldn't necessarily be the factor to go by.