They'll probably wait, but keep in mind that Part 7 and 8 are really long. Really long, as in, they might need 75 to 100 episodes to adapt, each.
They'd be around the same length as previous seasons. Part 6 has 158 chapters while Part 7 has 95. The average chapter length in Part 6 is around 30 pages, while the average in Part 7 is around 45.
Keeping in mind the first 25 or so chapters (and many following that) of Part 7 had around 30 pages (before the move to the monthly publication, Ultra Jump), we only have around 40-50 "big chapters". The really long chapters JoJolion regularly features are not as consistent in Steel Ball Run.
Then there's the fact that tankobon for Part 7 and 8 don't feature as many chapters as tankobon from previous parts. At least, reflective of my copies.
Steel Ball Run is definitely longer than previous parts, but it's not nearly as long to necessitate 75 or 100 episodes. Not even remotely close to that, in fact. I'd say 50-60 is a safe estimate, but I'd consider that pretty extreme, considering a lot of action in SBR is just pages showing ongoing events that do not require text; stuff an anime could gloss over in seconds what a manga would take several pages to show.