The length of the work permit you obtain will depend on a number of factors which are normally more related to your employer and your job/project than the strength of your own candidature. Our general advice is that you should not worry if you get only a short term permit as it will normally be possible for it to be extended if you are still needed by your employer (and if you are not needed by your employer, you may well be able to find a new UK employer who can get another permit for you to stay in the UK).
After 48 months physically in the UK on a work permit, you will usually be able to obtain permanent residence. For this reason, it used to be the case that work permits were not issued for longer than 4 years. However as many work permit holders need to travel a lot, permits are now issued for up to 5 years, at the end of which time most people should have spent the required 48 months in the UK and be able to get permanent residence.
Start up companies will generally not be granted work permits of more than 18 months duration; these permits are extendable, but copies of the first year's audited accounts will need to accompany the application for extension.
If you are to work at a client's site, your work permit will usually be limited to the duration of the contract existing between your employer and that client, but if the contract is subsequently extended an extension of your permit will probably be possible and easy.