I personally recommend dedicating half a day to ascending and descending Bluff Knoll, assuming you'll likely want to stop and take in the view at several points on the climb up, as well as spending a bit of time taking in the inspiring view from the summit.
That said, there is a Strava segment for the ascent and descent of the mountain, with the current record standing just over 50 minutes to the top and back down, here's the current leaderboard as of the 20th of February, 2020.
I'd estimate an average time with a little stop here and there for a fairly fit individual to be about two and a half hours.
You definetely don't need to be some kind of athletic specimen to climb Bluff Knoll, however on the way up you'll find yourself taking reasonably large steps onto wooden sleepers all of the way up which will put your quads and glutes to the test - as a test of fitness I'd recommend testing yourself walking up and down a large/steep set of stairs (if you're in Perth I'd recommend none other than Jacobs Ladder).
I'd also recommend foam rolling your quads, hamstrings, glutes and calves and performing a few hip flexor stretches before beginning your climb (or before driving to Stirling Range National Park) trust me, if you don't stretch your hip flexors your body will let you know that you should've afterwards.
Cost to Climb
As the name suggests, Stirling Range National Park is a national park, and as such attracts a $12 entry fee.
This is a trust system, you won't find someone waiting to take payment in a ticketing booth, you'll find a parking meter in the carpark to make your own payment.
I've climbed Bluff Knoll a handful of times and have never seen anyone inspecting who has bought a ticket and who hasn't, but don't be the guy that doesn't pay entry - the money goes towards maintaining this hike as well as others in the nearby area.
Buy the ticket, do the hike.
Take at least 2 litres (half a gallon) of water per person climbing.
Temperature/wind/rain etc. can be somewhat unpredictable during your climb so dress accordingly - I recommend wearing layers.
In terms of food I wouldn't recommend packing anything too substantial - some trail mix, apples, bananas and perhaps an energy gel or two will more than suffice. Simple carbohydrates will give you the energy you need to conquer this one.
Stay on the trail and keep an eye out for trail markers along the way, there have been a number of missing persons on Bluff Knoll and the Stirling Ranges National Park in general - be cautious.
You don't need fancy hiking boots but I'd recommend you at least wear some trail running shoes, the descent is quite steep and has a decent amount of loose terrain - the chance of sliding down or rolling an ankle if wearing regular runners is quite high (many people do it but I'd never recommend it).
Here's the sign you'll find confirming you're in the right place! Located at the path at beginning of the hike from the carpark in the national park.
This photo was about 3/4th of the way to the top, on this particular climb I wore a moisture wicking singlet, a long sleeved thermal over the top and a goose down jacket. I've regreted not wearing layers on previous climbs as I was either freezing cold or boiling with a super elevated heart rate.
The Summit of Bluff Knoll
Here's the summit sign you'll see moments before the amazing view over the ranges.