30-45 days, secure

Tell them one month or 45 days, it is secure. If he needs more, then he got to prepare a visit schedule (a list), such as dateS, visiting companies' names. Normally, according to my experience, for a business trip, if he tells them 3 months, he has to give them very good reasons. Anyway, you can "bargain" if they don't give you 3 months, though it takes a little bit risk. That was what I did.