Every major route (Britney, Keisuke, Rich) is planned for about 20-25 hours of reading (Rich on the short end of that, Britney in the middle, Keisuke on the long end of that).
Currently, Britney is about... 6 to 8 hours? It's hard to get an accurate length without just turning on auto read and timing it because C.H.E.A.T.S. makes (word count * average English reading speed of 250 WPM) estimations impossible from the micro branches everywhere in the script.
The re:Dreamer in-VN game takes place over 6 months (October 7 2022 to April 6 2023) but there's an epilogue for each route I don't want to get into spoilers here as that was not what was asked, but it will use time skips to very briefly cover the rest of what happens at the 3.5 years of college for Zach (or Zoey) and the route partner, a little after graduation, and setting the stage and tone for what this character's life is going to be like from now on and their trajectory into the future.
If you do want to heavily spoil yourself for Keisuke route, this outline and this outline show an excessively detailed plan over the main game and the epilogue which you could use as the framework for understanding how Britney's route would be presented over that same period of time.
tl;dr 20-25 hours planned planned for her route, 6-8 hours of which currently exist.