Often/usually the professional athletes complete their 6 month service in two parts (6 months being the minimum time, and usually the professional athletes never stay longer than 6 months), which could mean for example doing 3-4 months this summer and the remaining 2-3 months next summer. Though according to ESPN Lauri has said he wants no special treatment, but remains to be seen what that means.
Utah's Lauri Markkanen told ESPN he is confident he can fulfill his mandatory service in the Finnish military "in a way that it's not going to affect my preparation for next season."
www.espn.com
Effectively many professional athletes doing their service in the "Sports-school" aka "special reconnaissance force" treat the army time as an extended training camp. They learn how to use the personal weapons, learn how to stand in attention and march in step, but other than that the time spent in the army is pretty much a tough physical and technical training period spesifically tailored for the athlete in question (with some activities also targeted for all the sports-school recruits), and also a chance to to spend time in an enviroment where basically everybody are highly competetive athletes (many, for example Kimi Räikkönen and Mikko Koivu, have mentioned it was actually the best part of the service, to see how tough other athletes from other sports actually are, and how similar the tought-process for the professional athletes seem to be no matter what sports they are coming from). At maximum the "sports-school" accepts 80 recruits every year, and they are essentially all olympic level or near olympc level athletes.