Are there any restrictions on how long someone can be on a fixed-term contract? How often can you extend?

There is no restriction on the duration of a single fixed-term contract. 

Under the Protection of Employment (Fixed-Term Work) Act 2003 (the “2003 Act”), employees may not be employed on a series of fixed term contracts indefinitely. Where an employee has been employed on two or more continuous fixed-term contracts, the total duration of those contracts may not exceed 4 years. If the term of the successive fixed-term contracts exceed 4 years, the employee will automatically be deemed to be employed on a contract of indefinite duration, unless there are “objective grounds” justifying the renewal beyond the 4 year period.

The 2003 Act provides that objective grounds must be based on