Tis true, I've had several in one sitting in fact. By the end, not much milk comes out in the letdowns, but I guess its the more fatty stuff.
I seen guidelines that say express for 2mins once dry or express for at least 20mins.
Personally I've found I get milk out for about 12mins (albeit slowly near the end) and then get another letdown at 18-20mins. I think I slowly built up the time I was expressing for (if I was uncomfortable I stopped), also sometimes I feel like there is another let-down trying to work its way out (silly I know) so, I give it 5mins otherwise pack up. It seemed to be a long road in getting there though, with time of day and regularity being factors. (Before I expressed seriously, then I paid no attention to these things and just stopped when I felt like it)
Time (your free time) comes into of course.
I imagine like everything, every woman has to find her own rhythm. I think, in the whole, there doesn't seem to be alot of information out there for expressers. It seems to be a muddle your way through affair.
Maybe somebody with more energy than me, than set something up.
More of a ramble than advice but hth