OP - I don't think you rae being unreasonable and it's converstaions my DB and I are trying to have with my parents currently - they've had a holiday home in France for 12 years and 5 years ago they sold up the family home, bought 2 flats and a small 2 bed house, renting out the 2 flats keeping hte 2 bed for themselves. Now they are properly retired they want to rent out the 2 bed house too and live in France full time, coming back "for a month at a time" and "staying with people".
I have spelt it out, we have 2 DCs in a 3 bed house, I do not have a spare room, and putting one on an airbed in the other DCs room is something I'd do for 1 night, not a month. DB has harder issues, as he has no DCs and a 4 bed house, but he doesn't want them to stay for months at a time, and his DP has as good as said he'd leave!
My parents also have these rosy ideas of us all going to their holiday house for a holidays, but I've been once since having DC1 and DC2 hasn't been at all, it's the middle of nowhere, which is what htey wanted, but an 8 hour drive once we have crossed into france isn't practical with 2 small DCs and the only airline that does the route is Ryanair, is very expensive and we'd need to hire a car with car seats at the other end or be stuck in the house with them all week - it's a holiday we priced up at nearly a grand, it would be more once DC2 needs her own seat on the plane, to go somewhere that's not really very touristy so there's not much for small children to do. We'd only go as "duty" visits, not for actual fun.
DB feels the same, he's not been for 8 years and with limited holiday allowance, just doesn't fancy it. We were discussing that we feel we should go, but out of sense of duty, not after a 'cheap/free holiday'.
My parents have claimed htey plan to stay in hotels when they come back, but we'll see, my dad has deep pockets and short arms. I do think their 'plan' is to stop with DB.
It's sad, they say they love spending time with the DGC, and make me feel guilty for not making them available whenever they are back, but they are making a choice to do something that makes life harder for everyone else.
I dread when they are at old age, not sure how we are going to do any sort of care for them via a different country, or if they will come back and need looking after. Which seems unfair compared to say, PIL needing care later one when they are around more and help us out now.