How can I make one cell keep adding once another cell reaches maximum of
8:00.
Im trying to calculate overtime in a seperate column after employees enter
Start/Finish times from drop-down cells.

Assuming your hours are in H1, you can use a formula something like this to
track the OT hours...

=IF(H18,H1-8,"")

You will have to adjust this (the 8) to account for how the hours are stored
in H1; but conceptually, that is probably the approach you are looking for.

Rick