Payback days for training generally go in the bank. If you are a reserve unequaled it's not likely you would get payback days anyway. Training has always been very lucrative to me and friends. Read and understand the contract. Uses the tools on how to bid and manage training ALPA provides. My FO last time I went to the school house failed to do that. It cost him dearly. He had 3 shadow days going into the following month. He could have bid and held a 12 day trip touching the shadow period. With Greenslips being awarded that month he could have had a 200 plus hour month for working 12 days give or take a day.
I did not have the luxury of long trips but still easily generated 220 hours pay for the two months plus 3 payback days.