@johnb974
I would suggest that being inside the store and in your car are different things. The entire reason that there is a time limit in the first place is that some shoppers cut corners on doing the interactions properly and tried to get in and out as quickly as they could. The MSC and client put the minimum time requirement on the shop to ensure that the shopper would not cut corners - to dissuade shoppers from cheating. By leaving before the time has passed, you are flaunting the requirement. (If you don't like the requirement, don't take the shop!!) Every time a shopper flaunts a requirement (especially if s/he brags about it!), the MSC and client have to look for ways to make the shop more fool-proof - which makes life on all of us more difficult.
I, too, have had shops where the minimum time required for the shop was ridiculously high. I stuck around browsing for the required time. Then, in the "notes" section of the report, I made a note to the editor explaining that the minimum time was excessive for anybody who was helped immediately. Sometimes, the MSC takes this to heart and changes the requirements. Sometimes, however, the time requirement is in place because that is the amount of time the typical customer at that shop spends in the store. For instance, if you are looking to purchase a new shirt and have taken the time to walk into the store, you are not going to leave after five minutes - you are actually going to be browsing the shirts and looking through sales racks, etc. By not doing this, you are creating a scenario that does not mimic the typical customer and, therefore, not able to give the client proper feedback as to how the employee interacts with you. Moreover, you are in danger of outing yourself as a shopper.
So, yes, what you are describing cheats the client and the MSC. Moreover, it burns all of us by pushing the MSC to make more obnoxious proofs of visits than they would otherwise want. (Really, the client does not want a selfie of you in front of their store. That is put their to stop cheating!) In this industry, our integrity is everything. If you can't follow the guidelines and feel you can alter the guidelines to your own preferences, you should not be mystery shopping.
Hard work builds character and homework is good for your soul.