Really? Thats an interesting way to take it...I've never thought about taking it that way. I try to make sure it hits me in the thick of the day, so I try to take mine between 11am-1pm, then it wears off about 5pm-7pm I guess is when I start to feel less awake.
When I first started the medication, my doctor suggested this method in order to find the right dose. He prescribed me 60 30mg pills the first month, so I can start with 30 and go down or up depending on side effects. Basically, mix the powder with a somewhat precise measurement of water (I always do 1 cup) and drink a fraction of it.
Shire (the pharmaceutical company) says this is acceptable, but they say that the unused portion should be discarded immediately. I keep the unused portion in the fridge and it's fine. I've never stored it suspended in water like that for more than a day, so I don't know how long it would keep.
It's worth noting, however, that the powder does not dissolve in the water (I guess because of the chemical properties that make it extended release). So you need to shake it well and drink it immediately before the powder settles.