The bigger your chunks are the more you'll get done. A 1-2 hour chunk is not enough time to load and "sort" a new, sophisticated task with many pieces into working memory. If you've "sorted" the task before and stored it to "disk", sometimes you can load it in ~30m IME, but that still doesn't leave you much time to work on it.

Schedule an 8 hour chunk and expect 4-6 hours of solid work to get done on average IMO. Depending on the problem and how prepared you are for it, sometimes you'll go far longer than 6 hours not wanting to have to save and restore the task again.