The speaker of the poem begins by saying that if he and his beloved had enough space and time, he would not mind her coyness (her "playing hard to get"). If they had all the time in the world, he would not mind if she continued to refuse his love until the conversion of the Jews. (In Christianity, this conversion would occur at the end of the world.) The speaker then goes on to suggest other metaphors of how he would wait for her to return his love and he would continue to praise her in the meantime:
An hundred years should go to praise
Thine eyes, and on thy forehead gaze.
Two hundred to adore each breast:
But thirty thousand to the rest.
So, "long love's day" describes this long period during which the speaker would hypothetically wait for her to accept his love, in the physical and/or romantic sense. The speaker believes that she loves him and is hopeful that they will be together in every significant way, eventually. So, when he says "our" long love's day, he personifies the love between them. Their love, personified, has a temporal existence which he calls a "day" but "day" is metaphoric. In describing how he would wait for thousands of years, "day" means an extended period of time. "Day" is the lifespan of their love. The speaker gives their personified love a hypothetically endless lifespan.
In the last part of the poem, the speaker makes the case that they don't have all this time, so he implores her to embrace their love and make the most of this fleeting time: their "long love's day." Being less metaphoric and more realistic, the speaker suggests that their love's lifespan is short; not so "long." Personifying love, the speaker gives it human qualities: having a lifespan and a conscious existence in space and time.
Time is a crucial element in this poem and it is also personified.