This statement evokes careful precision: Prufrock rations his life in slight doses, not allowing himself joy or excesses. It speaks to monotonous repetition, particularly in the context of the preceding lines:
For I have known them all already, known
Have known the evenings, mornings, afternoons
Prufrock speaks both of constants—his life is allotted by coffee spoons—and of uncertainties: the “decisions and revisions which a minute will reverse.” One comes to understand that, for Prufrock, uncertainty is itself a constant.
There is an element of forethought, as well: "I have measured out my life" is ambiguous time-wise. Prufrock might be saying that he has already measured out his life in the past, but also that he has allotted even the future portions of his life—in other words, that he does not intend to change his habits.
Furthermore, coffee spoons symbolize the social rituals that Prufrock so dislikes: the “taking of a toast and tea” that obstructs any real conversation. Topics are bite-size and confined to suitable topics, not of the sort that Prufrock would prefer. He is a man who wanders narrow streets at dusk and sees a reflection of himself in the “lonely men in shirt-sleeves” who smoke pipes and lean out of windows—Prufrock must tire of the restraint, yet he too restricts himself.