How to evaluate equivalent 'samples per second' from Time base value?
Many scopes have different memory depth settings, but ignoring that possibility..

Assuming 10 divisions (pretty typical) it appears that your scope has a memory of 2500 points.  Therefore the sampling rate is 2500 / (10 * time per division).  For example, at 5 us / div the rate is 2500 / (10 * 5u) = 50 MSa/s.  And at 100 us / div, the rate is 2500 / 1000u = 2.5 MSa/s.  See?

So at 100 ms per division, the rate would be 2500 / 1000m = 2.5 kSa/s and at 250 ms per division 1 kSa/s.