I'm thinking of doing some day chasing south from my location into Iowa or Illinois on good chase days with someone around here. The reason I don't think I'd ever like setting aside a couple weeks at a time is that I could waste a lot of money and net a lot of nothing. Since I'm in closer proximity to good chase territory, day chasing would probably be more plausible for me. So its a tough call for you.
I don't know what everyone will think of this idea...but perhaps set aside two one week periods and space them nicely over chase season so if one half of the season is poor...you can still possibly nab something on the other end.
Some tidbits...
:arrow: 44% of all US tornadoes occur in May and June, with May being the most active month and June being the second most active.
:arrow: A paper from Mark C. Bove at FSU says that during La Nina events (which we are experiencing) the Ohio Valley and Deep South typically experience much above average tornado conditions...and from practical experience, La Nina events have coincided some of the more notable severe weather outbreaks, not just in the OV or DS either.
I would probably agree with Brian and you, Laura, that late May into Early June might be your best shot to get a tornado, but it seems to me that most major outbreaks occur before May 15.
Oh well, in the end its your call but if I was in your position I would consider a week from like April 28 to May 4 and a week like May 27 to June 03...something like that.