I hope you are well.
While there is not a current way to accomplish an alert only triggering at a certain time when conditions are met out-of-the-box, there is a potential work around that would provide similar results. You could create an alert condition that triggers when your log count is less than 50, then create a muting rule that will allow a notification at 5:15 PM. This will not stop it from opening an incident but will stop it from sending a notification during the rest of the day.
I also think that this would make for an excellent feature request, having alerts active during specific time windows. Would you be able to provide further detail surrounding your use case for this so that we have plenty of information to relay to our engineers? It is much appreciated.
We hope to hear from you soon!