You can set a Daily, Weekly or Monthly Deposit Limit by going to My Account > Play Responsibly > Deposit Limits or you can do this now by clicking here. You will also see the options on the Deposit page when making your first ever deposit.
A Deposit Limit is the maximum that you would be able to deposit during the selected time period, so you can still decide to deposit less if you want to. There's no subscription, so you're never committing to spending a certain amount of money.
Kindly note, when setting a Deposit Limit, it will take into consideration any money that's been deposited already that day. For example, if you set a limit of £50 a day at 5pm, but you have already deposited £50 or more, you will not be able to deposit any more funds that day.