Indeed, I stop thinking about them and if they write to me again and I'm still available, good for them.Exactly or they don't get back to me at all. If the person isn't asking when I'm available or giving me specific dates I'm assuming they're not serious now. I just don't understand what is the point of doing this.
What I started doing a few years ago is offering to secure that window for them if they send me a deposit right away, that way they can take a few hours or days to confirm their plans. If they can't see me after all, I keep the deposit as it's possible I might have declined one or more invitations for the same window.
This offer was taken a few times, with some clients forfeiting their deposit as their plans changed. I don't mind since my time was compensated.