When I started playing in Season One, games usually didn't go to 60 minutes---although they would occasionally, but I will say that only a very few ever went less than 30 minutes. Most of my games averaged around 40 minutes. Laning phase was typically 15 minutes or so (now it's about 7-10 minutes at most), "Mid game" was from 15-30 minutes and then "late game" was 30-45+. Surrenders happened, but they were also pretty rare (maybe about 10% of the time) and even then people would wait until about the 25-30 minute mark to /ff.
In my experience, games started becoming shorter in Season 5, I remember writing posts about it then, but they became much shorter in Seasons 6 and 7. Here in Preseason 8 I have yet to have a Summoner's Rift game go longer than 28 minutes. In fact, my last 10 SR games have gone 18, 22, 20, 19, 22, 28 (27:55 but I rounded up), 21, 23, 27 and 20 minutes respectively. I used to have ARAMs that went longer than that.
There is absolutely no doubt in my mind that Riot has tried to increase the pacing of games by adding more damage and snowball mechanics---this is essentially indisputable. Dragon now awards buffs that generally favor snowballing (damage to towers, movement speed or pure damage) and few that help to stall out games (bonus regen being about the only thing that MIGHT help to turtle a bit). Baron is even more of a siege buff and, frankly, so is Elder Dragon. Towers are a joke and with ever increasing damage and ways to shred towers even faster there is really no way to logically argue that Riot has not (either intentionally or otherwise) increased game pacing and, thus, decreased the length of games.
I saw a post from Riot that said game length this preseason is down only about a minute-and-a-half, but I have a feeling that there's a fair bit of deviation in there biasing the mean by quite a bit. Furthermore, that data doesn't pass the eye-test. I have noticed a SUBSTANTIAL decrease in game length.