It's good. I usually expect a minimum of a week when I commission an artist (level of complexity of the commission and the skill level of the artist obviously factor in, modifying this expectation). That said, I've no real expectation on any artist to finish it in a specific timeline (and in several cases, artists have completed nothing for me at all). Many things factor into such an equation. Personal emergencies and crisis's as well as other unforeseen circumstances obviously must be given allowances. One of the worst, most despicable things you can do as a commissioner is ignore the situation and stress out an artist even further in such a case because your commission is "taking too long". If it had meant so much to you, or you needed it by a set date, then talk to the artist before hand.
That said, I tend to be rather laid back and unconcerned about such things, as it's very rare I commission a work with a specific deadline in mind. So perhaps my expectations in such matters aren't quite the same as you'd find in many other folk on deviantArt. Just my two cents on the matter. ^^
(And yes, your week to two months is most reasonable.)