While the money may be one of the biggest things you need in GTA Online, it doesn’t give you the needed skills or tools to get funds and protect your wealth without the best guns in the game or gear that you needed so much so you spended ingame money to get it.
Reputation (RP) in the criminal underworld gains you respect and – more importantly – the fear of your lessers. It provides pride, dedication and lots of playing time to achieve the wickests levels in reputation points.
The big story behind “longer the missions = the more RP you get” isn’t just a general guideline for how much you might earn – it is a literal mechanic in the game. Funnily enough, the less time you spend in a mission, the less RP you get. Being solo is the best when doing missions, however Several missions are actually relatively easy to do alone on Hard, even if the recommended number of players is 2. Solo missions are also good for the following technique.
Basically, since missions doenst kick you automatically when you’ve completed an objective once you have done it and completed the mission, you shouldn’t complete the mission yet. Stretch your legs, make a cup of coffee, play with your dog or something while the 15 minutes pass. Its always good to have a break some time to time.