From bug 51262 comment 9:
> 1. Begin with a timer interval of 5s.
> 2. After throwing away a backing store, record the time interval required for the next paint.
> 3. Update the timer interval to 100 * the recorded time interval in 2.
> For fast-painting sites, this will throw backing stores away quickly. For slow-painting sites, this will approximate a maximum thrash cost of 1%.
We should do this! Right now we always throw away after 5s.
Note some of the complexity in defining "the time interval required for the next paint" mentioned in bug 51262 comment 41.