Hypothetically, it can sometimes take as little as five hours. But considering it often takes two hours just to enter/exit the Kathmandu Valley, it’s safer to call it a seven-hour trip. Nine isn’t unheard of.
The name ‘Sherpa’ has become synonymous with the people who can be hired to carry your bags while trekking or mountain climbing, but this isn’t really what the word means. Sherpas are an entire ethnic group from eastern Nepal, originating in Tibet hundreds of years ago, and concentrated around the Everest region. Many Sherpa people work as guides, porters and lodge-owners, but many do not. Sherpa is a frequently heard surname of people who belong to that ethnic group.
The Thamel district is worth basing yourself in to sort out the logistics of your stay in Nepal, as it is where the majority of tour agencies and hotels can be found. But it is unlike anywhere else in Kathmandu, or the rest of the country. Every shop is either selling souvenirs or tours, hotels are crammed together, it’s noisy at night and it’s easier to find pizza for dinner than typical Nepali food. For a quieter, more ‘local’ experience, consider staying in Patan, Bhaktapur or Boudhanath instead.
Kathmandu is one of the most polluted cities in the world, per capita, being on a par with notoriously dirty cities such as Delhi, Dhaka or Beijing. Dense population and poor road, water and garbage disposal infrastructure are to blame, along with the government’s apathy and inability to fix things. Yes, Kathmandu has its charms, but its cleanliness is not one of them.
Sure, you’ve had a tough day hiking in the mountains and you deserve a beer. But altitude plus alcohol can equal some nasty hangovers, or worse: alcohol can hasten the effects of altitude sickness. Better to save those rewarding beers until you’re down the mountain. (They’re cheaper at lower altitude, too!)
Due to a shortage of breathalyzers, Nepali police will usually just stick their noses into drivers’ mouths to sniff for alcohol. Worst job in the world? Not necessarily. Drivers believe that scoffing mints will fool the cops, so at least their breath is usually minty fresh.
Dal bhat (lentils and rice). Every time.
That would be the load shedding, i.e., a euphemism for the scheduled power cuts that cripple Kathmandu for up to 15 hours per day. Nepal doesn’t generate enough power to meet demands—especially in over-populated Kathmandu—so electricity supplies are ‘shared out’, with different neighborhoods receiving power at different times. Load shedding schedules are a favorite topic of conversation among Kathmandu’s residents, so join in!
It’s futile. Walk out slowly but confidently and the vehicles will go around you. Just don’t try that with an oncoming bus.
Admittedly, this usually works in Nepal’s favor (not as hot, not as crowded, fewer touts, safer for solo female travelers) but it really irritates Nepalis. Nepal is a completely different country, with a different history, ethnic composition, landscape, cuisine. Sure, India and Nepal are neighbors and share many cultural aspects, just like Northern and Southern Europe do, but comparisons are pointless. There are plenty of reasons to enjoy Nepal on its own merits.
The highest mountains in the world are certainly a major attraction of Nepal, but there is much more to the country than that. On your second trip to Nepal, you’ll know better: head to the jungle national parks of Chitwan or Bardia; learn to kayak on the Trisuli River; go on a multi-day white-water rafting trip; or visit rolling green tea plantations in the east. 
This trip was sponsored by ATTA.