How long did it take you to get to 600GB? Take that number, then figure it'll take you 50-75% as long to double it, and then doubling that will take about 50-75% as long as the first doubling. Any changes will really only go in the direction of it taking LESS time as games get bigger and bigger pushing the limits of the hardware.
Depends if you are talking about size limit or lifespan. If it's size limit, you know better than anyone how many games you intend on buying. If you're going to buy enough games to fill a 3tb hard drive, money is not an object to you. At about 10gb per game, your hdd can hold 300 games. 300 x $50 = $15,000. If you're talking about lifespan, the average life of a platter hdd is about 6 years. You will want a second hard drive to backup the first because reinstalling all those games if it dies will be a huge pain.