“Fascinating,” said Snape, without looking at it. “You should drink that directly, Lupin.”
To me, this quote seems to indicate that - since there was no immediate danger of Lupin turning into werewolf right this moment - there was a another reason for Snape to insist on Lupin drinking ASAP.
The only logical reason seems to be if the potion is more potent when drank right after making. But it could also be Snape just being snippy.
Based on both Slytherince's and Anthony Grist's answers here, it is 100% clear that the potion still works well enough for several days later.
Is there any canon confirmation (books/JKR info/Pottermore etc..) other than the PoA quote above of whether the "drink that directly" was based on the fact that freshly brewed Wolfsbane potion is more potent - though not critically so - than non-fresh one?