higher reaction time equals higher conversion rates to the more stable Δ8 isomer, if you don't mind this you could extend the time by 15-20 minutes and probably reach conversion rates that touch the 80% mark, but keep in mind that most of the Δ9 isomer that will be present at the 15 minute mark will degrade to Δ8-THC if you extent the duration of your isomerization, and it will not degrade just to Δ8 but to other cannabinoids as well, like Δ10-THC, HHC (mostly to the S isomer for Δ9 ), and of course to the stable CBN, Δ8 converts to CBN at time-exteded and high temperature isomerization methods, the outcomes are numerous, always depending on the reaction techniques and the reactants being used.