Out of experience, which species flowers' bloom for the longest period(individual flowers, not blooming season/flower number)?

Not sure if this helps, but I have noticed that if I keep the room cool and do not allow pollination, some of my Easter cacti (all hybrids) will keep the same individual flower for more than 2 weeks. I've never timed it exactly by individual flower, though, only by "bloom season" which is still not accurate because they can sometimes bloom sporadically all year.

I am curious to hear more answers to this question. I realize you intentionally excluded short-lived flowers with a long blooming season, but this plant spends about half the year in bud or bloom, and is the most floriferous cactus I have seen.

Parodia schumanniana

Baja_Costero said:this plant spends about half the year in bud or bloom

Just curious is this "sporadic" bud/bloom (i.e. different amounts here and there) or is it a big flush of blooms like other cacti, but lasting six months?

The flowers last 2-3 days and appear in groups of 8-10, on average.

Baja , that is intentional. Because, you know , mammillaria and schlumbergera.
my experience(could be wrong on some of them)
opuntia humifusa -up to 6 days, depends on size and conditions
thelocactus setispinus - 3 days tops, only one time it bloomed
gymnocalicum baldanium - 2 days,average
schlumbergera x buckley/truncata - 4 days (i think) max
parodia- longest ive seen is 4 days
Rebutia -1 to 2 days
echinopsis - 1 to 2 nights(if lucky)
disocactus x hybridus - 1 to 4 days (previous owner)
opunita elata- 2 days
disocactus flageliformis - 2-3 days maybe?
lepisimium cruciforme -3days, could be longer
lepisimium bolivianum - 1 or 2 days
schlumbergera gaertneri -4 to 5 days
rhipsalis - 1 day

Those straw flowers on the heads of cactus sometimes last for years (sorry, couldn't resist)