No rule of thumb, as tri-pedaler said, it all depends what the next element is. I will say I found that I started going to 3rd more willingly, if I could get into it earlier. These motors make max torque at what 4500rpm? So in a section that I maybe on redline for only 1 second, but if there is a section I can go to 3rd 1-2 seconds before I would hit redline, then I am in 3rd for 3ish seconds pulling hard, it was worth it.
The big thing, when you do 3rd, you will be going faster into that next element, so your braking points need to back up to match.