There are a number of bus companies that operate regular services throughout the day from Phnom Penh to Siem Reap. An alternative way to get there is to fly, which will definitely reduce the journey time but will likely increase your costs.
Duration: 2-5 hours
Some companies offer overnight sleeper buses, but most offer hourly or two-hourly services running from 7:30 am.
The bus should take around 2-4 hours, however this can vary depending on the company that you go with. We travelled with Paramount Angkor Express, a company that I would not recommend.
This is actually a local bus that runs from Phnom Penh to Siem Reap which, by no means has the Wi-Fi or on-board toilet as advertised. The journey with Paramount took 8 hours (which, compared to the quoted 2 hours is a crazy difference!) and our washroom break was actually just in the middle of a field. Be careful with baggage too and keep your valuables with you at all times.
Other ‘Getting Around’ Guides
Getting from Phu Quoc to Sihanoukville
Getting from Sihanoukville to Koh Rong
Getting from Kep to Phnom Penh