How can one know if vaginal discharge stops regularly or not, in order to determine how to act, especially with the feeling of it descending sometimes after ablution (wudu') or changing clothes?

These discharges are pure; they do not necessitate changing clothes or performing istinja', but they do nullify wudu'.

If you know a specific time when they cease, sufficient for purification and prayer, you should wait to pray at that time.

However, if you do not know a specific time for their cessation, you should perform wudu' after the time for prayer begins and pray the obligatory (fard) and supererogatory (nawafil) prayers with that wudu' until the time expires, disregarding anything that comes out.

The decisive factor is whether or not you know a time when the discharge ceases, sufficient for purification and prayer.