Higher volume SST intervals, what is to much?

My current plan includes one day of 1hr SST blocks (one a.m., one p.m.) in which I am targeting 85-90% FTP, another day of shorter intervals (12-30 min depending on the week) where I am targeting 100+% FTP, again a.m/p.m workouts. Sunday is my day to get in long rides, but I really want to keep it focused to working on building what my focus is for the specific phase of my traing. So initially they will be more SST based. These rides will progressively get longer and harder as I progress through my respective phase, all of the way out to 5+hrs. This week was only 2.5hrs, being that it was my first week and my goal was to sustain 85% FTP for a solid 2hr interval (I managed 87%). My question is given my other workouts during the week am i better served extending the length of my time riding SST or is there a point where breaking things into 2 intervals with a short break makes sense during the real long days? My original thought was to continue extending the length until i hit a point where I am doing such long intervals that I can't maintain 80%. Truthfully IDK when that will be, I simply have not done many steady sustained efforts beyond 1hr.