I did some more number crunching with the rest system, and noticed the two constants which are most important:
1. the base of the logarithm
2. the reduction factor per minute
The reduction factor - how many percent of rest the character loses in each minute where it does something exp-worthy - is most important to define the "cutoff duration" - the length of a play session after which the exp bonus becomes negligible. Due to the exponential decrease it doesn't matter much with how much rest a character starts out. This affects the duration just by some minutes. A reduction by 1% makes it possible to play from dawn till tusk until the bonus falls below 1.5%. A reduction by 2% gives you an afternoon of good exp. With 3% reduction, it's about an evening. When I put it up to 10%, the fun is over after less than an hour. When I set it to a whooping 50%, you burn through your exp bonus in 10 minutes.
a lower base of the logarithm makes the rest system more effective overall, especially after longer breaks. The lower the base of the logarithm, the higher the rest bonus a character starts with after a break.