Vegas's right. Approximately two seasons is a rough estimation. What I do is the following:
-Trainer lvl 4 (no speciality), hired with 12-13ks salary & keep him until his salary is around 20 ks. Then fire him and get another one.
-Doctor lvl2 (in my case with "career extension" speciality, but it's a matter of choice) around 4-5ks. Keep him until his salary is 10-11 ks.
-Public Relation Manager lvl 2 (in my case with "crowd involvement" speciality). Same as doctor; hired with 4-5ks and fired at 10-11 ks.
Doctor and PRM shouldn't be very expensive (max. 15 ks each, I'd say). The trainer is a bit more complicated, and now with the season start everybody is looking for them (I'd pay between 10-50 ks for a trainer like the one I told you).
At least that's what I do. It doesn't mean it's the best option, but it works fine for lower-middle division teams
Hope it helps. Good luck with your team!
there is no career extension doc. they do have massage docs, who can help if you have too many minutes..was that what your saying? or was it taping injuries? anyways I like trainers with career extension, PR Guy with a National Appeal (I win more road games, more attendence for the home games get it?) then a doc with massage in case my best player got like, let's say, 90 minutes.
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