DD got £100 a month, which paid for toiletries, non-basic clothes (so we'd pay to ensure she had a couple of pairs of shoes, a couple of coats, jeans, underwear, a few basic tops), presents, going out, treating herself to a drink from Costa, etc.
At 17, we upped it to £200 a month, which also covered all clothes, transport to school (quicker train or cheaper bus) and lunches on school days school (unless she made herself lunch from home).
If she'd got herself a PT job (she didn't as she was very committed to working for her A levels and did a lot of study for them), we'd have given her the same. Having said that, if we were in a position where the above money made an impact on our ability to spend on other things, we'd not have been so generous and she'd have had to either cut down her spending or get a job.