The limit will depend on your mobile operator and your phone plan ; although, you will be applied your domestic rate, it will only be on a set data volume. The extra data will be charged at a rate that should not exceed 4,50€ per Go which is the European wholesale data roaming cap. This rate is valid for 2019, it will decrease at 3,50€ in 2020, 3€ in 2021 and 2,50€ in 2022.
If you have unlimited data on your phone plan, your operator will calculate the fair use volume as follows : 2x your monthly mobile plan rate / wholesale data roaming cap (4,50€ in 2019). So,if your phone plan is 20€/month, you will be allowed 9Go/month while outside of your country of residence. Note that this is the minimum allowed, your operator can be more generous.
If you have a limited data allowance on a very cheap mobile plan (less than 2,25€/Go), your operator can apply a limit based on how much you pay. Regardless of the situation, your operator must notify you when you sign up and when you reach the limit.