Hi Colin - not a cyclist, so I'll let other folks who are address that. I know that many here have returned to competitive cycling, either as part of a competition (triathlon,etc.) or in competitive cycling.
Personally, I do high impact sports (tae kwon do, soccer, basketball) and the usual gym stuff (weights, yoga). At three months I could do none of those, although in PT I was on a stationary bike for about 20 minutes.
My limitations were no weights above 50 lbs until six months, no impact sports until one year, running was limited, not by my surgeon but by my personal experience until about nine months. Some have returned to running after six months and were successful and did fine.
The main reasons to avoid impact in the first six months is 1) to let the bone grow into the device and settle well and 2) to avoid the neck breaking before the six months mark.