This should be fun to see I expect him to do fairly well at least in his first few matches simply due to athletic ability and aggression, he is also at a very good gym. James Haskell joins Bellator as heavyweight MMA fighter
Looks like he's got at least four years of training in already: Hobbyist - Professional Rugby Player James Haskell | FIGHTLAND
Clubs have worked with judo coaches for a while for the same reason, Wayne Larkin worked with Leicester Tigers for example, shoot used to get a fair few academy players visiting, that was fun. How much actual training he has done, given how hard in season training is for rugby, then international commitments etc is anyone's guess
With his level of athletic ability 6 months is a lot of time, we have seen what D1 level footballers with little training can do let alone elite level athletes. And don't forget a lot of rugby is about staying on your feet, in general play and in rucks in fact thats why they brought judo guys in to help them with clear out techniques etc
I was joking man...."6 months sprawl training" is basically the joke formula for any non-mma athlete to succeed in high level mma.
Well it looks like he's been training, even passively, for quite some time. He'll do well especially since he's heavyweight. Wish him all the best.