this is known as custom and practice but there is certainly no specified period for which it must be in place for it to turn into a contracted term. It could be months, it could be years - it is not just as simple as saying it has been in place for a defined length of time - there are many other factors that are relevant.
Case law has suggested that the following are important factors when considering whether a term has become implied in a contract:
· On how many occasions, and over how long a period, the terms in question have been applied - the more times they have been applied and the longer the period over which this has occurred, the stronger the argument they had become implied into the contract
· Whether the terms are always the same - large differences will make the argument they had become implied weaker
· The extent to which the terms are publicised generally - there must be widespread knowledge and understanding amongst the workforce that such terms were being applied
You may tell the employer that you believe the term or practice you are relying on has been implied into the contract through 'custom and practice' and see what they say. They could of course deny that and refuse to discuss the matter and if that is the case then you can only realistically challenge this by taking your case to an employment tribunal, although you are free to raise a grievance with the employer first to try and deal with this internally.