Hardy or tropical hibiscus?
Tropicals are better grown in smaller containers and moved up one size at the beginning of the next season if the rootball filled the whole container. Couple this with a low or no nitrogen liquid fertilizer and you will get spectacular blooming.
Hardy hibiscus need large to very large containers because, well, they are fast-growing large plants. Especially during the hottest Summer months the rootball will grow like crazy while the plants blooms like crazy. Some cultivars like the Suniq series can be container-grown, but they are better planted in the ground.