You should pump the grease until the boot starts to swell a little bit.

If you pump it until it fills up, or starts to ooze, you run the risk of pressurizing the boot and causing it to pop or come loose. It is common for old boots to leak before they swell because of holes. Just do what has to be done then.

If you have already put 8 or 10 pumps in it, it may need more but it may be a bad gun, a bad fitting, or it could be swelling on the side you can't see.

If I feel like I added enough grease without a noticeable movement of the boot I put my finger over the gun tip and pump trying to hold back the grease. I have at lease 1 gun in the basement that will squirt grease but not build pressure. Then I remove the fitting and slowly pump through the fitting. Not uncommon for the fittings to be blocked or bad. If both pass the test I pump in some more grease.

I will put right angle fittings on many joints, if it will clear the headers and such, for ease of greasing.

Also I always remove the boot and look at the joint before I install it. It is very common for brand new grease-less joints to be bone dry of grease. No wonder they wear out in a few years or less.

Lastly if it is a grease-less joint already installed, I will use a grease needle to inject grease under the edge of the boot. Sometimes under the edge isn't an option so I puncture the boot with the needle to grease them. Only if I feel like it needs the grease though.