I filter by recently online. Pick chicks who were active in the last two days. Shotgun any interesting candidates. Do the same next day, etc.  Pipeline 'em.

There is no rule or deadline for any of these. She will respond when she responds, or not at all.  
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Yes, in general a faster reply means a higher level of interest from her. But she's wading through her own mountain of messages from POT's and trying to sort out the scammers, rapists, poor-as-fuck wannabes, and a-holes from the rest. She may already have 4 M&G's setup this week, so she slow-rolls you for a few days. She may be a pro (or semi-pro) and she's got several bigger wallets in line ahead of you. She may be on travel somewhere (with another SD) and just isn't ready to put you in here queue. You may be on her B or C list because of distance/your looks/your attitude/or whatever.   Really, it doesn't matter and you shouldn't care.  
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Just do your thing, send your messages and read your replies when they show up.  
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Life is good.
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The Cat

because as Herb said, there's no consistency on that score.  I  had  a fantastic year long arrangements with a gorgeous 22yo who did not agree to meet me for 2 full years after my initial message to her.  OK that's a bit extreme, but it happens.  I've often had conversatio9ns lead to nothing, then get picked up months later and lead to BCD.

One thing I've found worthwhile is to sort the search results by newest and send the attractive newbies a gentlemanly message of introduction right away.  Often they get swamped with lots of scammers and weirdos and get so freaked out that they delete their profiles quickly.  I had that happen to me very recently but since I was one of her very first correspondents, and very possibly the only non asshole in the bunch, she sent me her number before deleting her [profile and we've been texting offsite.  I'm supposed to be meeting her for coffee tomorrow.

The new SA policy to delete messages after 6 months (noted in older posts), means it's a bit of a challenge to pick up with a POT who you have not yet connected with off the site.  
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I've been looking at recently active, but not new, profiles and seeing quite a few who I messaged a long a time ago.  Of course, I can't see the last (or any) of the older messages so I lose some of the continuity I'd like when resuming communications.  Not a serious problem, really. But you do need either a perfect memory, or you need to take a softer approach to restarting the chat (Hi there. Saw your profile is active and thought I'd check in and see you are doing...).  
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I suppose you can manually archive your message threads with a simple copy/paste to your PC. But you'd be keeping a lot of threads that will ultimately be useless just to save the one or two that could be useful. Add if she changes her profile name, it could be hard to even find the thread you need. Unless you also saved her pics.... which I do.  
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Life is good.
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The Cat