In order to answer your question you almost have to answer the questions you didn't ask. First of all, the engine sounds for planes are the ENG, boats BENG, vehicles VENG. The engine sound itself is probably best to be a minimum of eight seconds, and no more than twenty-four seconds. If it gets too big it can cause problems on systems that are using onboard sound, or USB headsets. Pretty much everyone knows how I feel about that already. If you have listened to any of mine you can hear a pitch shift in some of them. On those sounds you need longer tracks to make it work effectively.
Engine startup should be eight seconds, but I think in practice it varies from plane-to-plane. It is tough in some cases to choose which eight seconds, because engine starts can have some really good sound and usually go longer (on up to three minutes) to really smooth out. So, my practice is to record and mix the engine sound I want and save it to AH. Start AH and run the engine at idle while I record AH playing the sound. Then I mix the engine startup into the idle sound. You can run the startup a little long then so that the transition is as smooth as possible.
Engine shutdown is usually eight seconds, but I notice some of mine are ten seconds. I mix the idle into the shutdown so it's smooth.
If you need to know a precise time, just record a given tone as your sound and then record the playback from AH.