Take a look at the current coalition, most governments will only get 5 years to implement their ideas and policies this limits the government to short term policies as the next government might undo their work. Take Gove's new national curriculum, it would have been much easier for the teaching community if it had been over 10 years but instead was rushed through in a couple. Furthermore look at the economy how is a government supposed to revive it in such a small time. Policies are always best done over a long-term period and our current system simply does not offer this.
Anything could happen even within a single year that could completely devastate a country. If the current prime minister is unready or not suited to tackling that problem, then we would be sitting around helplessly for ages until we can finally vote of someone that can actually help the country. Therefore, making terms longer could spell disaster for a country. Instead, I propose that we could increase the maximum number of terms. This way, someone cold serve as prime minister for many years, but if at anytime the public decides it's time to change, the poll wouldn't be too far away. If possible we could even drop the whole rule on maximum number of terms.
What if certain world events change that the prime minister doesn't specialise in, or people change there opinion. Then we could be stuck with them for a long time. Shorter amounts is much better, so different prime ministers can focus in different areas the country needs improvement on. We should have a labour and a liberal prime minister.