Lavatory Drain Stub Out length?

How far should the 1-1/2" PVC drain pipe stick out of the wall before the 1-1/4" trap adapter is glued in place? 6 inches? Couldn't find this anywhere. This is for a vanity cabinet sink.

There is no spec on this. 6" would often be too much, interfering with proper alignment of the trap. You would want it to be generally close to the wall. If the vanity has a back, which some do and some don't, you need to be sure the trap adapter nut is accessible. You could just stub the pipe out long, and after the vanity is in place, cut it to the right length and glue on the trap adapter.

It depends on how deep the cabinet is. If you are unsure you can always stub out long and then when ready to trim cut it to the desired lenght. However I usually cut the 1-1/2" flush with the finished wall and then install the trap adaptor. After the cabinet is install you can only see the trap adapter threads and the escutcheon will hide the connection.

It can stick out as far as necessary so that the 1 1/4" trap will reach it. But, we usually use a 1 1/2" trap and reduce to 1 1/4" at the drain's tailpiece.