I completely disagree. If times where for example shorter then if you did a robbery in Paleto it would take a cop from LS roughly 3mins to arrive on scene.
As for a realistic stand point i have personally read & seen Store robberies from professionals can take roughly 5mins for a quick cash & grab. But since you make anywhere to around 5k-25k 5mins is good enough. Now if you want the time lowered then i will lower the price you make as well. B/c 9/10 when you rob a store you would only be able to make out with around 1k and for big stores that are frequently shopped at you would make around 2k. The reason being is in those cases you are only taking whats in the Cash Register.
In order to get more you would need to gain access to the Safe. Which is where you would normally get around 5k.
I know this mainly b/c i have worked for Grocery Stores & Small Gas Stations.
The reason for time being 5mins is b/c i calculate the time to Rob the Cash Register & the Safe.