It's really a gamble to pick which time of year to go. Let's go year to year (recent few years)...
In 2003, if you went to the plains in the first two weeks of May, you'd score...
In 2004, if you went during that period, it primarily sucked (with the exception of 5-12-04 in southcentral KS) and the last 10 days of May rocked...
In 2005, if you went anytime during May, severe weather was quite scattered and good days were far between...
In May 2005, the only notable days were the 10th (in southern Nebraska (nice mothership supercell, but thanks to high LCLs and lack of low-level shear, no tornadoes)), the 11th was decent for the very few who waited along the dryline in western Kansas, the 12th was incredible for those who played with the high-precip sups in the TX panhandle, and the 13th carried a nicely striated supercell in northwest TX as well.
In June 2005, everything changed... You had quite a few good tornado days, including the 4th (hard to find the tornadoes, but there was a few very nice ones), the 7th in South Dakota, the 9th in Kansas, the 11th and 12th in the TX panhandle and the 29th in MN (and a few other days for a few other chasers).
Me and Kurt Hulst set out towards the plains on the 27th of April, for a 3 week trip. We started with the setup in AR/MS and chased a high-precip supercell into northcentral MS afterdark (yeah, we needed a bad storm, really bad) and then set out towards OKC, only to meander around the area for the next 10 days, stormless. By the time my first "vacation" was over, I had gotten about 5 tornadoes and several nice storms. But, it wasn't enough, so I went back out again in early June for an additional two weeks (and got about three times the amount I did in the first half).
I guess you can say the longer the time you have available to chase, the better (well, obviously) as you will have more setups to chase. In my case, I have plenty of time... LOL :lol: