I am allowed to work up to 20 h per week. In the first year I managed to squeeze a few hours here and there in the evenings and weekends. Since I started my field surveys I barely have enough time to do my laundry or take a shower, so I have worked very little, maybe a couple of weekends since October. I am aware that in the summer I will have to do some work and hopefully earn some money. I think it depends on how intense your phd is, and how well you do with multi-tasking.