How long before my 60-day visa stamp expires should I apply for an extension?

Go a few days before and they'll give you 30 days from the end date.

At least two weeks before expiry is good deal

With VOA you would most likely get 30 days extension from a day when you go there.

So, if you want as many days as possible - go there the day before your date on the stamp or at the last day as your stamp is showing.

I prefer going one day before the date on the stamp, because anything may happen last day (unexpected situation which wouldn't allow you to be there on time, unexpected to you out of office day or so...).

Can you only extend for 30 days and what happens when the 90days is finished ? Can you extend again

what type of visa do have?

You can extend it any time before your 60 day stamp expires. I extended mine today for 30 days and I have 6 days left on my old stamp. It gets extended from the stamp end date, regardless of when you do it.

Within 30 days at most offices but within 7 at Jomtien and Samui