Sites like mircscripts.org or Hawkeye have a lot of scripts. I don't know if they have something like what you're looking for. If not, maybe someone here is willing to write up something, but usually people here will help with problems someone has when they are writing their own script instead of writing the entire script for someone.
As for a general idea of how it might work, you might just have a script echo lines from channels/queries/etc to hidden @windows and then every time a /timer triggers, /write it to disk and clear the window and repeat.
Keep in mind that any method to delay writing of logs can, and most likely will, cause lost data if mIRC crashes or quits without having a chance to write the stored logged data beforehand.
What is probably a better option is to just not log any channels that you don't need logs of. Stick to logging only important channels, which in most cases don't have a lot of text coming in. This helps to reduce hard drive access because you're not updating logs very often if the channels you log don't include a lot of text.
