TCP established timeout (for new connections) can be set using command:
set system conntrack timeout tcp established 28800
Normally , this is only a couple of hours. On my setups, I try to please NAT users and up it to 1 day like command above. Try setting value to 4 ....8 hours.
Normally. TCP session are closed nicely (FIN and FIN-ACK packet), if firewall sees those packets, it will remove entry from table immediately.