I can only speak from a Type 1 point of view:
I had to provide 90 days of blood sugar readings (3 to 4 times a day). Once I was approaching the end of those 90 days, I had to get bloods done for my latest A1c etc, eyes screen for retinopathy, feet checked for any nerve damage and a stress ECG carried out.
This is was all submitted to the CAA on the 90th day. Two days later I was invited down to Gatwick to have consultation with the CAA specialist whom signed me off. I got lucky with dates to see the consultant as he is only in a handful of days each month.
I was off 4 months on total once my airline got me in the sim for a medical test flight with a TRE.