Prostate Cancer UK produce a leaflet called "Treating erectile dysfunction after surgery for pelvic cancers", with the sub heading "A quick guide for health professionals: supporting men with erectile dysfunction".
For Tablets (PDE5-Is) (sildenafil, tadalafil, vardenafil and avanafil) it says "Early initiation (after catheter removal or within 14 days) promotes early recovery and preservation of erectile function".
For a Vacuum Erection Device (pump) it says:
Early use (within one month after surgery) linked with better outcomes. Can be initiated 4-8 weeks after surgery.
I had no problem being prescribed tadalafil and a vacuum pump, but various factors outside my control meant I didn't get the pump until 4 months after surgery. I had already bought and was using a cheap pump (£25ish from LoveHoney) and that worked fine and still does. If (having had an RP) you face a wait for an NHS prescribed pump then I would buy a cheap one to use now, bearing in mind the remark above about early use being linked with better outcomes.