I would suggest the G1000 is easy enough to pick up, but a lot depends on how familiar you are with systems of this type. I find those accustom to using computers all day long "take" to glass cockpits much quicker. Even so I would suggest 4 or 5 hours on the Garmin sim is required for you to be comfortable with the system for VFR flights while using most of its route planning and other functionality.
Adding command of IFR functionality ( and by that I really mean the ability to plan and fly IFR approaches and departures including setting up a different approachs or departures on the "fly") will take at least another 4 or 5 hours of your time and some real time use in the cockpit.
I think the "test" of whether you are really familiar with the G1000 is to have someone with at least 100 hours on glass sit with you, set up some scenarios and ask you to reprogram the system and call up some of the display options. If most of it is second nature then you have really got to grips with the system.
I found that the different makes of glass avionics are surprisingly similiar - I started out with the G1000 on a DA40 and DA42 but actually having mastered the G1000 the transition to other makes is surprisingly easy - even if on the odd occasion you find yourself doing it the Garmin way rather than Mr Avidyne's way for example.