in general the more proffessional your players are and the more high level your facilities are the longer players can go without first team football and the higher the level of competition needs to be for them to benefit from it compared to staying in your reserves.

if you're a top level PL team for example, likely your good 18/19 year old prospects will not benefit from L1 football. at that point you need to make sure they're playing first team football at a decent level if you send them out. you want to send them to a team with decent facilities in a second tier (non top 5) league. ideally they can also get some CL/EL games in as well.

first team football is what raises CA after 18/19 age but a good player on a team with super facilities if they go to a bad team in a low rep competition will likely make very little progress on loan.