In spite of the exultant happiness with which George and Emily leave the stage at the end of Act II after their marriage, one of the overwhelming messages of the play seems to be the ephemeral nature of such happiness by pointing towards the common end that awaits us all in death. If we have a look at the beginning of Act III and the introduction that the Stage Manager gives us, we are given the answer to your question:
This time nine years have gone by, friends--summer--1913.
Thus we are told that Emily and George have had but nine years to enjoy their love for one another and to make the most of the life that they were able to have together. The brevity of this time of course reinforces the central theme of the play, which is making the most out of the litle life that we are given.