Gran Turismo 3 - 100% completion 140 hours. This is a close friends completion time, I only get about 12% in the end. He beat GT3 100% in two and a half solid weeks of playing, approximatly 7 hours a day. Whoever typed 900 hours must have obtained every car, but to complete all the races and get 100% completion it's much less. I wouldn't consider purchasing every car as 100% completion. I don't know anyone who has maxed out GT1,2,4 or 5.
GT3 can be done 100% in such a time, as it didn't really had any long endurance or competition, rather than the Polyphony Formula cup. I don't remember my time, but it wasn't far from that as well. 900 hours does seem a bit exaggerated.
I also maxed GT4, but that did take me almost 1000 hours. Mainly because of the long ass endurances, repeated the Polyphony Formula cup a lot of times (it was much harder than GT3's cup), Mission 34 being a pain to gold, double the amount of events per license and overall huge amount of events.
I'm at 1% away from Platinuming GT5...but I just can't find it in myself to go back to GT5, especially since I have a big backlog of games to catch up to, and farming to level 40 from 38 A-Spec is going to take me a long, long, long time >_<.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"