On a circling you have min 400' obstacle clearance within a radius from the field according to category, f.ex. 4,2 NM for cat C aircraft. You are obliged to keep this until you can safely execute a normal visual descent to touchdown.
The obvious thing here is you cannot start the descent before being abeam threshold, and of course no later than outside the applicable radius. Every company has its own procedures.
A rule of thumb:
1:Start timing abeam
2: at 3xMDH(in hundreds)+-wind: Start a slow descend.
3: Intercept PAPI