Are you only talking to POT's who are online when you are? That's the only condition I can think of where this "online now" matters. IME, any POT that is online or was online within the last 2-3 days is viable.
It's not practical to expect a non-pro POT to be online and presumably actively monitoring incoming messages 24x7. It's even less practical to expect an SD to do so, since we have, by definition, lives/careers/business/etc. that keep us busy. Not to mention that an SD who is online 24x7 with nothing else to do than troll for a 20-ish SB is likely to come across as a loser to POT SB's. They want to meet successful, well-rounded, well-funded, interesting, and BUSY guys who are short-cutting the dating process. Not minimum-wage, married/divorced gas station cashiers who wish they could get laid. Not implying that is you, but what image are you projecting if you are online too much?
Actually as I write this, it occurs to me you could be looking for a quick pump and dump hook up. So online POT's are needed to go from message to text to BCD in minimum time. Nothing wrong with that. But is it limiting your potential to build a stable rotation of fresh, sweet pussy available for prescheduled or on-demand BCD?
Next to (at last) address your actual question; I suppose the best time to search within a 24-hour cycle depends on who you are trying to find. If it's college students who are likely in class from 8 am to 4 pm, then afternoon to late evening is probably optimal. If it's low-wage workers who a probably in retail or food service, then perhaps late night (after 11 pm) to late morning is best. If it's MILF's who want to step out for a Daddy Fling, then perhaps late morning to early afternoon (say 9 am to 5 pm) works as the Hubby is probably at work and the kids are probably in day care/school.
So perhaps you are best qualified to answer your own question by considering what type of SB you are most interested in meeting (note: "meeting" = fucking

.
I think we would all appreciate hearing back from you if you spend some time testing various time slots and can report any significant findings.
Life is good
The Cat