How long will a 30x20x8 inch enclosure last a young ball python?

[Bogertophis]

First off, snakes grow at their own rate- depending on how often & how much they eat.

30" x 20" is quite large for a hatchling & would probably take at least several years for a BP to outgrow that, if not for the 8" height, which (IMO) is too low for any BP other than a hatchling (for maybe up to 6 months). Obviously I don't know the orientation of your enclosure (access & ventilation) but by the time you add substrate & hides, that is a very low ceiling, & might easily be a safety issue for the snake, depending on how you're heating it. It's hard to answer this without really seeing it.

In the past, I've used a couple enclosures that were 12" high & I hated them because the side-opening doors (which were much less than 12"- roughly only 8" or 9" at most- sorry, but I'm working from memory) were awkward for me to reach in & clean adequately, or reach in for the snake, for that matter. (And I'm not a large person either, btw.) If this enclosure of yours fully opens on the top (which I very much doubt) that would help you with maintenance, but would still be awfully low for the snake- even snakes that don't climb much like to stretch up & climb around some, & a low ceiling invites nose or head rubbing & injuries to their face (that tend to turn into abscesses requiring surgical removal because snake "pus" doesn't drain- it's solid, not liquid).