Innerestin' -- the only plants listed as having a GROW_DUR tag (in v0.31.12) are all five of the "wet cavern" biome plants. Plump Helmets and Pig Tails are 300, and the other three are 500, whatever that stands for. It's certainly not days.
At the very minimum, at least 500 of these 'ticks' have to go by in a single season. Maybe there are 1000 ticks / season, and the quick ones mature in a month, while the longer ones mature in a month and a half. Just guessing there though.
Iceball3 is right: Growdur represents the turns that plants take - 100 normal frames (as compared to most creature tokens using a 10 normal frame average), and there are 1200 frames per game day. 12 Growdur equates to 1 gameday. (Covered fairly extensively in the Improved Farming thread, by the way.)
Plump Helmets and Pig Tails grow in 25 game days, all others default to 500 Growdur, or 41
2/
3 days. (And all game months are 28 days long, so all seasons are 84 days long, which roughly equates to 1008 Growdur.
Anyway, in 40d, even if a crop is growing, and a season where it shouldn't be able to grow comes around, it should instead create a "⌐" symbol for a dead crop, not simply an empty field.
If crops, especially crops like Plump Helmets or aboveground crops, which should grow year-round, dissapear, then we're talking serious bug. (Note that, while it's not exactly a feature, there has been a long-standing problem with ALL crops dying at the start of a new year/change to Spring, even if they are year-round crops. Hopefully, this will eventually be fixed with the Improved Farming changes.)