A HIP remains valid as long as a property remains on the market. If your estate agent decides that he'll no longer act for you (as he's fully entitled to do), your HIP will continue to be valid as long as you're still trying to sell the property.
You could either immediately engage the services of another estate agent or, possibly better, try marketing the property yourself. (On the two occasions that I've sold a house I've avoided paying an agency fees simply by putting up some big 'For Sale' signs of my own, and paying a small amount for local advertising. Many people don't seem to realise that using an estate agent is similar to employing someone to do their decorating for them; it's a lot cheaper to go for a DIY approach).
Chris