I would imagine that for 18-21/22 yr olds that are college age, they would come over for a night here or there but I wouldn't have them staying there for 6 weeks straight. 

If they aren't going to college, why aren't they working? Is your DH cutting them financially off for not growing up? My kids know that they have expectations upon graduation.

If this is an out of state thing, I could see paying for a 1 week visit in the summer and at Christmas time for those 18-21/22 yr olds but after that it would have to end. It would be one thing if they were being responsible in life but for a general loser that can't figure it out, you have to draw a line somewhere. 

I'm getting ready to enter this stage, and I've been curious to how it will work, too. Though, I have told both OSS and DH, multiple times, at the same time, that OSS's bedroom is being converted as soon as he goes to college UNLESS he lives with us (supposed to live in the dorms, but that may not happen due to pandemic). He'll have a bed (well, a pull-out or futon) if he wants to visit overnight (he's a decent kid, I'm not going to stop him from a weekend visit), but it won't be "his" room anymore.

I say take a similar approach. The minor kids have space; the adult kids have guest accommodations with guest timelines. If adult SK wants to visit for a week, sure. But once they are past their first summer after high school, they need to stop associating your house as their house. It may still feel like the familal home, but they need to start finding their own way in the world, and a permanent landing pad isn't going to accomplish that.

What does he want to do? Once a child has aged out they are an adult an visitstion doesn't exist anymore. You aren't required to do anything. 

I think there are two issues here.  First, whether a parent should pay for an adult child to visit and second whether a parent should enable an adult child to be lazy and fail to launch. 

I have no issue with a parent paying for the adult child to visit.  In fact, if the parent wants to see the child, that might be the only way it's going to happen.  My parents pay for my family (me, DH, bio and 2 skids) to join them on vacation (they have a  beach house 20 hours away).  We are very appreciative and make sure they know it.  But they want us there and there is no way that we would go if they weren't paying.  Going on vacation with my parents is not a priority in our budget, even though we love going.  We just can't afford it.

I do have an issue with a parent paying for an adult to spend 6 weeks with him where the kid has failed to launch and won't be working.  A week or two (a typical amount of vacation) would be fine, but 6 weeks?  That's just enabling the kid, because he should be working or going to school or otherwise being productive for that 6 weeks  I think having the kid come for 6 weeks, even if the parent isn't paying, is also enabling, unless the kid is planning to get a job for the time he's visiting dad