Very good topic.
For me, warming up is essential. How long it takes to warm up is a function of how embedded the style is in my brain/hand. Some styles only take 10 minutes to warm up. Anything for weddings in the copperplate or Spencerian family are 5 or 10 minute warm ups.
If I needed to do a piece in italic for an exhibit where I needed to show my best work, I would probably take two or three days - maybe longer - to brush up on my italics - and warming up -once I got to doing the piece would take longer. I can toss off an italic hand on an envelope, but I'm very rusty with broad edge.
Your window of 60-90 minutes sounds about right for how long you can stay focused. If I warm up for 10 minutes, I can usually work for an hour - plus a little. But after 90 minutes, I need to take a break. Once you get the style embedded in your brain/hand - your warm up time will shorten significantly.
I know some scribes who can write for 8-10 hours a day. For me, I can only get about 2 good hours in the morning and then another two hours in the afternoon. Maybe an hour in the evening. If I have to really push and put in a solid 7 or 8 hours of work to meet a deadline, I can do it. But the following day I know I will be worthless at producing quality work. Or I can push for 2 or 3 days in a row - and then I will need a couple days off.
If I had intended to make calligraphy a full time career, I am sure I could have done the work to build up to the 8 hour days, but I had no motivation to do that. Most people are squeezing calligraphy into busy schedules. I know many teachers who consistently offer this advice:
If you can put in 20 minutes of practice - every day, during the week - you will do better than just splitting that 2 hours and 20 minutes into 1 or 2 sessions on the weekend.