a non etops aircraft (class A, mtom>45360kg and mapsc>19) must fly on route not further than 60 minutes from a suitable aerodrome.
for class a with mtom<45360kg and mapsc<20 the limit is 120 min or 180 min (subjected to approval from the authority
turboprop class b and class c aircraft may fly 120 minuts or 300nm, wichever is nearest to the alternate.
riciprocating class b may fly 90 minuts or 300nm.
all the time and distance are intended with one engine inoperative.
i think this is the answer to the initial question. it's a general rules applicable in any phase of the flight except for take off alternate and isolated destination
ciao ciao