It will depend on a number of things. House, location and price.
Price - the condition of the house is always a key factor when looking to sell your house. Many sellers aren't property expects and so will always rely on the estate agent to give them a sensible valuation. This is where the first issues can sometimes come about as the estate agent my value your house high to win your business. It's always a good idea to see what houses in your area sell for, not what they go on the market for. You can do this by looking at
http://www.rightmove.co.uk/house-prices.html and typing in your postcode. If your price is too high then could spend a long period of time on the market. So make sure it is priced to suit your market.
Location - as the old saying goes it is all about location, location, location and this is still true today. For many, the open market can be a viable place to sell your property. However, some areas of England/Wales have very slow markets. Areas such as Blackpool and parts of the North East are known for their low activity. So it might be worthwhile looking on
http://www.home.co.uk/ to see the average time houses in your area spend on the market. If a short time me it might be worth trying a higher price. If a long average time then try to be competitive with your pricing. Again, if you are buying where the house has spent a while on the market then try a low offer.
House - Some houses spend a long time on the market due to their internal condition. Cluttered houses tend to not sell as quickly. This is a good article from
http://www.thisismoney.co.uk/money/mortgageshome/article-2840815/Sarah-Beeny-s-tips-want-sell-home-carpets-good-photos-online.html If you are stuck with a house that you can't sell then there are always a few tips to get rid of it quickly. Auctions are always a good option and most auction houses will give you a free valuation. One of the bigger ones is this one and
http://www.auctionhouse.co.uk/ and if it sells at the auction then you will normally expect a completion within 28 days. You can also try cash buyers like
www.homehousebuyers.co.uk who will offer you a sale in 2 weeks normally.
If you haven't had any offers on your houses within a the first month its time to address the price or the agent. Also try stay away from signing long contracts with your estate agent.