After 48 months physically in the UK on a work permit, a sponsored candidate can usually obtain permanent residence. It used to be the case that, for this reason, work permits were not issued for longer than 4 years. However, as many people posted to the UK are now required to travel regularly, and thus do not spend 100% of their time in the UK, work permits will now be granted for up to 5 years (the hope is that, after the 5 years, your employee will have spent 48 months in the UK and thus 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.
Permits for staff to work at a client's site will be limited to the duration of the contract existing with the client. Applications for any extension will need to be accompanied by proof that the contract has been extended (or that another contract has begun).
It used to be the case that, only in exceptional circumstances, or for board level staff, would work permits be issued for an initial 4 year period. Now it is Work Permits (UK) policy to issue for long periods wherever possible – although staff working on client projects are still restricted by contract length. However if a work permit expires, and you have a continuing need for the candidate, it is now very easy to obtain an extension to the permit (the need to re-advertise the post on extension has been formally scrapped).