As "g3uiss" has advised, it's very much dependant on OR's resources in your particular area regardless of the ISP you have chosen.
With FTTP already being available in my area for close to a year, I placed an order with BT (migrating from Zen) on 01/02 this year and received a provisional install date of 24/02 which unfortunately never went to "confirmed" state by OR, the order somehow ended up cancelled then re-instated by BT with new install date of 24/03, this date was confirmed and the fibre install went ahead.
In the same area, my son's FTTP UG installation took close to 3 months with many visits by OR and their sub-contractors due to duct blockage issues.
My daughter's FTTP installation, again UG, all went completely to plan within 3 weeks, all be it, the CBT chamber is in her front garden!
So, it appears to be very much luck of the draw regarding available resources at any given time in a particular area and any possible logistical complications that could arise!