I think it's 50 days in Belgium, may be less for Croatia.
Worth saying that you could probably get their permit before 725 days. I loaned two players out to my Belgian feeder club. One player was a great prospect and I was fortunate enough that even at 18 he was in the first team for his nation (Tunisia). I delayed it a bit because I wanted to loan him out for experience anyway, but when I offered him a contract after 2 years (the rules for Belgium are 3 years, rather than 2) he got a work permit without any trouble.
The second player was a bit stranger. 20 year old Aussie, playing regularly for the under 21s/23s , but no senior caps. After his second season on loan he returned able to play for us. When I loaned him to my Scottish feeder club he was approved for a work permit. May just have been a bug though.
Take home message is that when the player returns each year you should offer him a new contract to trigger the application process.