It got changed a while back, at least on the outer planets, to I'd say about 30+ mins average on any one tree, but I doubt anyone really knows, because probably nobody sits and watches individual trees, let alone enough for an average.
On thick spawns, if you cut away on an area recently untouched, it takes a while to get back to the start spot anyway. If it is not fully regenned, then ofc returning 20 mins later will see trees respawned, just mostly not the ones you just cut.