You're right! Always use real stopover length (RSL) i.e. the period between first capture and the bird leaving the site, rather than a minimum based on ringing data, as you say :) 

Hi Michelangelo,

I recommend you to read the following paper:

Guérin, S., Picard, D., Choquet, R., & Besnard, A. (2017). Advances in methods for estimating stopover duration for migratory species using capture‐NDASH‐recapture data. Ecological Applications.

Cheers

Hugo