How far do you put the endmill into the Dewalt router?
Until nothing but the cutting edge shows. Sometimes I’ll leave a little more sticking out if I’m cutting deeper into the material.
If it’s a stepped end mill where the cutting diameter is smaller than the part that goes in the collet, then I stop at the step.
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And if there’s a collar, stop there… 
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could you put it out further if you go a lot slower?
Instead of slowing down, take a shallower DOC, or smaller stepover. You need to reduce the lateral forces on the bit, but you don’t want to start generating a lot of friction by spinning the bit on the material.
You really, really need to consider if you have mount your bits shallow. The collet has a much better grip with the shank bottomed out, and the bit is less likely to slip/break/become a tiny projectile of hate and resentment.
Thank God… I managed to get through that post without a double entendre about Collette preferring it when your shaft bottoms out…
D’oh! 
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Do you mean bottom the endmill out in the router? I cant do that, it would swallow my bit whole
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True, but you do want the bit as far into the router as it can go, the collet gets a better grip on it, etc. As mentioned by David, you want as little of the bit sticking out as you can get away with. Now, if you’re doing some deep pocketing or profiling, you may not have a choice. Which is why they sell long bits. 
And technically, there is probably a limit to how deep is useful for grip purposes, and that may not be super deep. The other issue is that the further the cutter is from the spinning core of the motor, the more any runout will be exaggerated.
So you have to balance collet grip, lateral leverage on the bit itself, and runout exaggeration against the need to get more length out of your bit.
Keep it as short as you can, but if you have to, you can slip the bit out some. Just be extra super careful all the way around, and be prepared to lose a bit (or two). And like I said, you want to reduce the lateral load on the bit, but you want to do that via reduced DOC and stepover, not slower feeds.
You really want a majority of the smooth part of the bit in the collet. It’s not safe otherwise. Think about it this way, if you wouldn’t do it on a router you’re holding in your hands, don’t do it on the CNC.
20000 RPM is not something I’d be playing with. If you need more depth, buy a longer end mill.