The current regulations are below.
A joker engine is classed as any other engine though; once installed it must be used for 2 races.
28.4 For the purposes of this Article and Article 28.6 only, an Event will be deemed to comprise P3, the qualifying practice session and the race.
a) Each driver may use no more than one engine for two consecutive Events in which his team competes. Other than under f) below, should a driver use a replacement engine before the end of the qualifying practice session he will drop ten places on the starting grid at that Event and an additional ten places each time a further engine is used. Unless the driver fails to finish the race (see below) the engine fitted to the car at the end of the Event must remain in it until the end of the next Event. Any driver who failed to finish the race at the first of the two Events for reasons which the technical delegate accepts as being beyond the control of the team or driver, may start the second with a different engine without a penalty being incurred. An engine will be deemed to have been used once the car's timing transponder has shown that it has left the pit lane.
b) If a driver is replaced after the first of a two Event period, having finished the first Event, the replacement driver must use the engine which was used for the first Event.
c) Should a driver use a replacement engine after the qualifying practice session, he will be required to start the relevant race from the back of the starting grid in accordance with Article 36.2c).
d) After consultation with the relevant engine supplier the FIA will attach seals to each engine in order to ensure that no significant moving parts can be rebuilt or replaced. Following the first of the two Events, and within two hours of the end of the post race parc fermé, exhaust blanking plates (with one 10mm diameter inspection hole per cylinder) and further seals will be applied in order to ensure that the engine cannot be run until the second Event. These seals will be removed at 09.00 on the day of initial scrutineering at the second Event.
e) Other than the straightforward replacement of one engine unit with another, a change will also be deemed to have taken place if any of the FIA seals are damaged or removed from the original engine after it has been used for the first time.
f) Except during the last Event of the Championship season, each driver will be permitted to use a replacement engine without incurring a penalty the first time this becomes necessary during the season.