I don't know if they know at this point, and if they have long-term plans it could easily be changed if something else makes sense.
There are a few advantages to a real-time format.
It leans into the idea that it's been more than 20 years since Ultimate Spider-Man #1, so this Peter Parker is an adult version of the teenager many readers fondly remember (although in this case, time travel shenanigans changed his history.)
Ultimate Marvel was originally more accessible versions of the classic Marvel Universe, but now the more distinctive thing would be to keep going forward, to have this be the series where characters can change in big ways.
It doesn't seem that characters are tied to their age. Peter and MJ aren't going to be substantially different if they're in their mid-30s or their mid-40s.
It opens up new stories if characters get older, and comics can depict seasonal events.
This is also how the MCU does it. The characters age as actors do. And stories can reference holidays or seasons because they don't have to worry about the continuity mess of keeping these details vague in the regular comics.
I imagine they'll cheat often enough.
Sometimes a multi-issue story could cover the events of one day.
The MCU is mostly real-time, but Far From Home was set in the summer and No Way Home was the next few months, even if the films were two years apart.