Loss limits for betting start from the moment they are set. They run for the time you choose (week, month or year) and then they are automatically re-set and begin again.
As with all limits, they take into account the amount deposited during the previous period.
For example, if you set loss limits of £500 (€500) per week around Monday mid-day, your losses will be tracked from that point and you will be prevented from losing more than £500 (€500) until the following Monday. Then the loss limits will be re-set for the following week.
If you increase your limit, it will come into force 24 hours for UK residents and 7 days later for other residents. Decreases take effect immediately.