At what point would a suspension of an undeermined length (no definite length) turn into a termination? For the purpose of final pay.

When the employer reclassified it. This is not an issue that is covered by wage and hour law.

So, if I am an employer, I could give an indefinite suspension and not have to worry about "final pay" until the employee quits?

When an employee is dismissed from work and not given a definite time within the next pay period in which to return to work, it is a termination as far as immediate payment of wages is concerned. If the suspension is paid, obviously, it can last indefinately.