Looking at the achievemnts I'd say it takes about 5 hours to get most of these. Like if you really try, since these are really easy AND iirc you only need like 25 to unlock all 3 because its like 15 -> 20 -> 25 or so I have heard maybe this is just bullshit.
You could prolly get 10 of them in a single game and other 10 on a 2 player server with a friend. The medic achievements were bullshit since almost half of them required an ubercharge which is tedious to get as is while playing carefully enough not to lose or waste it.
EDIT:
Like seriously, most of these are about IGNITING something NOT KILLING.
- Ignite 5 enemies with intel, EASY
- Ignite 10 cloaked spies, PRETTY EASY TOO
- Ignite 10 zooming snipers, EASY
- Ignite 3 medics who have 100% charge, SUPER EASY
- Ignite Medic+Unit healed by the medic
And the list goes on, all you need to do for most of these is wave your flamer around like a maniac and you will most likely add up to a lot of the achievements. And these are mostly all pretty routine things too unlike with Medic where you need to kill 20 guys with a heavy, like come on who is capable of this if there is a single spy or sniper in the game.