How far in advance should i book my seat reservation.

Hi!! I’m going interrailing from the start of june to the start of august. my destinations include

-ghent

-krakow

-prague

-vienna

-budapest

-ljubljanna

-venice

-florence

-rome

-naples

i have an interrail pass but with that you obviously still need to book seat reservations. I’m wondering how long in advance i should book them or if i could just hop on whilst im there? i know high speed trains in italy might need more in advance but not sure how long? Also if anyone could give any insight into book the seat reservation with interrail process that would be greatly appreciated!