Extend to a point where you can reverse comfortably without getting shot. You may also want to extend far enough to gain an energy advantage through high speed and/or climb rate. Often very fast planes can climb at speeds that slower planes cannot match, even though in a straight time to altitude contest, the slower plane may win handily.
The answer is... it depends. I'll normally extend to about 3k or 4k before I reverse. I'll only do so if I think I have some advantage I can exploit. Reversing otherwise is just dumb, unless you need to change direction to rtb or head for friendly help. It depends on your speed, his speed, plane performance... etc. You want to be able to execute some sort of efficient reversal and set up to avoid being HO shot before you merge with him, so keep that in mind. If you reverse too early, he can follow and stay on your six, or at least have a nice deflection shot as you turn. If you wait too long, depending on aircraft performance, you may lose E in comparison to the enemy. Just make sure that you can reverse comfortably with some vertical element (up not down) and be ready and have enough speed to maneuver before he is in guns range. If you reverse and merge with him flying too slowly, he can take an easy front quarter or HO shot and you won't have the speed to dodge it. I like to leave enough room for an oblique immelman, a fairly gentle one, and set up another merge. Then I will either go for a hard angles merge (nobody expects me to do this in a Jug!), a pure E merge, or just run like hell.
