@Talking Gina
Yes, I said around 3 hours to complete the main story and about 6 to 100% get all the collectables, complete the missions and travel on all the alternate pathways. It might be a tad longer but most playthroughs I've seen takes 2-3.5 hours to complete the main story, which is a tad longer than Forces.
The time to beat the main story takes nearly as long as Sonic's half, but Sonic Generations takes far longer to 100% complete all missions and find all the collectables.