It shouldn't take long especially with Lewandowski. Btw but before going to the adaptation issue they now need to increase the price in their proposal to attract attention from Bayern's side because indeed the transfer issue is still uncertain right now why bother about adaptation issues not necessarily he will be in Barcelona too if they don't raise the price for the clause Lewandowski.
About him having to get used to his new style of play at Barcelona I'm a bit dubious. It will take a maximum of two games, especially in the first game he can show his level. More likely, if he gets signed he will understand the style of play already in pre-season training, has time to get acquainted with the team and so on.
Xavi's style of play is not much different from what Robert Lewandowski learned when Pep Guardiola was at the helm of Bayern Munich, I think it didn't take him long to adapt to the style that Xavi adopted. Although Lewandowski's transfer process is quite complicated, it seems that Barcelona are very serious about signing him. Barcelona are trying to carry out final negotiations with Bayern Munich for Robert Lewandowski, Barcelona will try to raise the final offer of around €40 million.