whats the longest springer front end i can put on a harley sportster?

Probably going to depend on where you live.
I know, sounds stupid, yet true.

When you extend the forks a lot, you’ve gotta increase the rake some to keep the front end out of the sky.
Some places have limits on how much rake is allowed.
As with most vehicle laws, this changes from state to state.

Springer Sportster by Sportster Specialty