If you want to keep your options open, an immediate reply is the best choice. If you have a copy and paste handy, it takes all of 2 minutes and you have a high likelihood of staying in the recruiters data base with adjectives like "decisive", "easy to work with", "good communicator", "future prospect".
Something along the lines of:
"Thanks for your message, I appreciate you reaching out to me since I
think XXX is really interesting company. Unfortunately I'm currently
fully engaged and not on looking for a new opportunity but if it's
okay with you, I'll keep your name on file so we can connect if a
future opportunity emerges"
If there is something specific you are looking for, you can certainly put this in there as well.
"I'm certainly interested in working at XXX but this opportunity
doesn't feel like a good fit I'm looking more for a senior level
technical role in web development ..."