I stumbled across a video on youtube where somebody gets 1 million roses in one sitting, taking about 11 hours and 45 minutes from start to finish:

The current record for the Unlock Rose Stage is just under two hours... Add that two hours to the 11h45m roses time to get a theoretical time of under 14 hours for a full 1 Million Roses speedrun. The current 1M roses record is over 16 hours, so was thinking the strategy in the video I saw could be used to lower the time by quite a bit.