Increasing the duration would be a matter of increasing how long the previous feeling can be sustained. They say that the best way to increase the targets of your affection is to love yourself completely. Apparently you can't give something you don't have. One methodology for expanding the subjects of love is to go from person to person, thing to thing, experience to experience and simply aim your love there for a while. The least desirable person's face can often be transformed by looking for the face they had as a child.
Compassion can be an eye, and heart, opener.
Everyone has loved, at least, one or two people simultaneously, yet each in their own unique and wonderful way. How many have tried genuinely loving everyone within a room, for example, or everyone in the city or town, the overall region, the state or province, the country, the continent, and on the entire planet. Find a spark of love in your heart for all things, every where, throughout all time, in every dimension and hold that thought for a while. Amplify it, say, ten times and sustain that for a while. Now amplify that by ten billion suns exploding.