Well I tend to be really chicken about these things
so I'd take it quite slowly, say, shave off 15mins from what they're currently doing and see what happens, then repeat... But if you want to go more quickly, I wouldn't go less than an hour just yet.
On the other hand, one thing I noticed with DS especially was that if I put him down too early, he'd mess about a lot longer and end up falling asleep much later - even putting him down just 15mins later could result in him actually falling asleep half an hour earlier than before, if that makes sense? Tbh I suggested capping the nap because I read the routine as up at 6am and asleep at 9:30pm, which makes for a pretty short night - but if it's usually more like up at 7:30am, nap for 1.5h and asleep by 9pm at the latest, having played around for 1.5h, then it's possible you could end up with a longer night by putting them down at 8pm rather than 7:30pm, iyswim? Do you feel that they prefer a longer or shorter night? Are they rubbing eyes like crazy in the morning or do they seem like they've had all the sleep they need?