What is the safe separation for landing a Cessna-172 behind a commercial airline jet?
It depends on the size of the preceding aircraft. ICAO PANS-ATM-Doc 4444 Air Traffic Management has some values.

5.8.2 · Arriving aircraft
5.8.2.1 Except as provided for in 5.8.l.l a) and b), the following separation minima shall be applied.
5.8.2.l.l The following minima shall be applied to aircraft landing behind a HEAVY or a MEDIUM aircraft:
a) MEDIUM aircraft behind HEAVY aircraft- 2 minutes;
b) LIGHT aircraft behind a HEAVY or MEDIUM aircraft- 3 minutes.

Note that in case of A380, it is more- 4 minutes.
In case of distances, the light aircraft are to be landed 5nm after a medium aircraft, 6nm after a heavy and 8nm after A380.
Note: The categorisation of aircraft is done as below:

4.9.1 Wake turbulence categories of aircraft
4.9.1.1 Wake turbulence separation minima shall be based on a grouping of aircraft types into three categories according to the maximum certificated take-off mass as follows:
a) HEAVY (H)- all aircraft types of 136 000 kg or more;
b) MEDIUM (M)- aircraft types less than 136 000 kg but more than 7 000 kg; and
c) LIGHT (L)- aircraft types of 7 000 kg or less.