A day has ~10⁵ seconds
This is trivial, but I make use of this all the time when doing some back-of-the-envelope calculation on whether some computation is going to take a few days or a few years:
A day has about 10⁵ seconds (24*60*60 = 86,400 ≈ 100,000).
So, if an operation takes 1 ms of CPU time, then you can do 10⁸ operations per day on one CPU and ~10¹¹ on 1,000 CPUs.