They are very critical as to placement so play around a bit a find the sweet spot.
They recommended to me that I only play a reed for about 45 minutes then give it a good rest. I have found that if playing constantly in that time its best to put on another reed and rest the first one for at least an hour. If I am only playing horn lines and a few solo's I find that I can do a 3 set gig with 15 minute rests between 45-50 minute sets with no deterioration.
I am on the same 2 alto reeds now for over a year.
I switched to the Legere Signature Reeds about 8 years ago, and at first found them very tricky to get placed right. It doesn't help that I play silver MPs on tenor, so the reed nearly vanishes at the tip, and you need really good lighting to place it. (This is especially true until you're familiar with the "sweet spot" as Jazzaferri so accurately described it.)
With regards to the length of time the reed can be played, I've found over the years that these reeds no longer have that 45 minute limit.
When I first started using them, I played in a rock & blues band, and I played and soloed A LOT. I barely got through a 45 minute set without my reed going so soft that I couldn't hit my altissimo notes anymore by the time we got to the last song in the set.
However, shortly after I started using them, something changed in the reeds. Now I no longer have problems with "Legere Reed fatigue" as I referred to it as,
I think I may have stated earlier in this thread, but in case I didn't: I use the reeds on all but one of my horns (SATB sax, as well as Bb, and bass clarinet), I have no problems using the same reed for the entire rehearsal or show, but do have 4 or more that I rotate between. Yes, they last a really, really, long time. (4 reeds/year). That said, I have had quite a few duds in the last couple of years, and at what they cost, that's where my hackles go up a bit.