exact conditions will vary from contract to contract... you would most likely end up in Africa unless you have 500+ hrs on a B200 in which case Afghanistan would be a possibility... 28 days is about right but whether you take it or not will depend on the location... due to the high costs of air travel to somewhere like Kinshasa it's possible that they'll ask you to stay in country for the duration but leave one month early on the years contract...
You might be better off applying to Naturelink who provide aircraft and crew to AirServ on a sub-contract basis but offer much higher salaries... it's been a contentious issue for a long time amongst AirServ high time skippers that the low-time FO next to them is paid considerably more ! One reason why I left. They'll tell you that they're an NGO and therefore a sort of charity but the fact is they bid for and operate exactly the same contracts as all the other operators. Not a bad company but shopping around before commiting yourself might be a good idea.