It's a pleasant couple of hours trekking along the Ngong hills.
Max altitude is 2460m, but you start walking from quite high up, so it's not as daunting as it sounds.
The cheapest you can get a taxi is about 1500Sh each way, plus about 700sh for loitering around while you walk. The journey takes about 45mins, but can vary depending on the traffic in Nairobi itself.
You will also need a ranger with a gun. The KWS have a small hut at the entrance, containing a variety of rangers, and a variety of guns... I got charged 1500Sh for the ranger. I suspect I was ripped off, as this is more expensive then Mt Longonot, but I'd still rather pay that and have a gun-toting protector than save the money and not... (They are needed partially because of the threat of buffalo, and partially the threat of muggers - In Oct 2006, some missionaries were literally stripped of every item the carried, including their clothes. They were offered a ranger, but wanted to save their money. Doh!)
You can either walk from the gate, or drive a bit further up to the 'car park' at the radio masts. The track is a bit rough, but if the taxi driver is happy to drive it, who am I to complain? Walking from the gate will add about 90mins total time to the walk, but would e quite nice. It's an easy track, with some nice views for a picnic. You can see (depending on time of day/haze) Mt Longonot to the North, Nairobi & beyond to the East, The Rift Valley Escarpment to the West, and (just) Kilimanjaro to the South.
The hills consist of 5 main peaks, and to get to the highest will take about 60-90mins, (The return slightly quicker) depending on fitness. The high altitude is noticable, but not debilatating (I'm not the fittest person in the world, but neither am I a couch potato).
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.