When is the latest date I can leave Thailand on a tourist visa if stamped until February 6th?

Face it you're not the first foreigner to get stamped in wrong AND not bother to look at their passport before they left passport control (the counter where you stamped in).

Your stamp clearly has ผ.30 written on it and that means 30 day visa exempt.

You can go BACK to the airport where you stamped in to the information counter and they will go get the stamp corrected OR you can go to the immigration office in the area where you're staying (in bangkok it's out at the government complex on Chaengwattana Road) and get it corrected there.

Of the two choices I'd go back to the airport because you won't have to queue up and they'll do it in just a couple minutes ..

Your msg is confusing, show us a picture of the tourist visa and the stamp you got from immigration won't take long to figure out what happens. You can blur wtv personal info. I say confusing because some people here are using wrong terminology, not saying you are, but need to be sure.