Florist chrysanthemums may last for a month or more in bloom indoors, but seldom remain attractive longer than that. If you'd like to keep the plant, I'd suggest transplanting it outdoors now, and mulching it in well over winter. Because mums are daylength sensitive to form their blooms, it may not bloom naturally until quite late in the season outdoors, and may not survive winter as well as mums bred specifically for the garden.