I always thought it was 1/2 and hour per 8 hours but what oyu are doing has to have an R&R allowance built in, usually around 10%.
I know for people who process where I work on a 7 hour day they get 30mins lunch breaks and around 5 mins to goto the toilet, get a drink etc per hour built into the target (it roughtly works out to 10% if they are sitting down and 13% if they have to get up and move around). That said these are only what my place allows as there is not a set rule for the amount of time to be allow for this.
I would suggest speaking either to someone in HR or a union rep (or CAB)