In the grand scheme of things the timing isn't critical (unlike fermentation temp for yeast, or cleaning and sanitising your gear).
Your temperature rise time will depend a lot on your gear, and how you ramp the temp. The rate will often be limited by you heat delivery method (heat exchanger/HERMS/RIMS/Direct fire/adding hot water)
I'd mash for 60 minutes (you're most likely using very well modified malts), then start your ramp (you can start at 50 minutes if you like). Once you hit your mash out temp, then start the 10 minute countdown. If it takes you 15 minutes to get to your mashout temp don't stress, it won't matter.
Again, total mash time doesn't really matter that much (unless you mashing for hours). If it's 75 minutes with a mashout or 92 minutes with a mashout there will be virtually no difference in the final product.
What is your planned method for raising the temp?
JD