It's been a long time but I'd say roughly..
MGS: 8+ hours first play, 3 hours speeding through skipping cutscenes. Few VR training missions.
MGS VR Missions / Integral / MGS PC: VR missions - lots more hours.
Twin Snakes: MGS minus VR training, think the 'temperature card' stuff was made less backtracky so takes less time. Never paid any attention to completion time.
MGS2: 16+ hours first play, never tried speeding through
MGS2 Substance: VR missions, silly story missions - LOTS more hours.
MGS3: 20+ hours first play, never tried speeding through
MGS3 Subsistence: Online play - LOTS more hours.
MGS4: 20-30 hours first play according to reviewers. Online play - Lots and LOTS more hours.
MGS2, 3 and 4 are packed full of easter eggs and 4 especially multiple ways of approaching each situation so there's lots to replay for. MGS4 also has some sort of game completion rewards system and the weapons/items/special unlockables etc you collect should carry over to the next play.
Thorough first play-throughs of course.
