I'd say a good gauge of how long you could possibly go is maybe 2-4 weeks than your longest hair washing stretch.
If your scalp is normally feeling icky, sweaty, caked with product/sebum etc after 3-4 weeks, you know 5-8 is your max.
The other thing is everyone's hair grows at different rates, sometimes at different times of the year. Have you ever dealt with 12 weeks of buildup/lint/shed hair? I'd say it's not for the faint of heart, and while it can be done with relative scalp safety and hair retention, I would definitely not recommend you shoot for that on the first go.
But like others have recommended, different steps can help out with making your stretch as bearable as possible. Probably the best of the recommendations for a long stretch: redoing a few rows every week after maybe 4 weeks. For example, if you've got 12 rows of twists, do the first 4 rows on weekend 4, the next 4 rows on weekend 5, the next 4 rows on weekend 6 until you've redone your whole head, then redo again.