Metal Gear games are really short. MG1 can be beaten in one sitting. MG2: Solid Snake takes a bit more effort but is it still short. MGS1 is like 12-15 hours. Replays send that number to 4-5 and that's with skipping cutscenes. The same is true for MGS2. MGS3 is on the long side. It took me a full 30 hours to beat when it first came out but that was due to my play style, as I am a perfectionist when it comes to Metal Gear.
Of course, these numbers are first run numbers and don't include replays, speed runs, pure stealth no alert runs, no kill runs, or extreme mode runs (extremely challenging). It's totally worth it though even if you just gun through each game in order playing them only once, particularly if you have not played MG2 or MGS1.