Hi there
I used to take St John's Wort about fifteen years ago when I was also going through mild depression and found it very helpful indeed. I took it for around about a year - I would say it probably took a couple of weeks to really get into my system but this is purely subjective.
But more recently my husband started taking it; it worked pretty well at first but then he started to get dizzy spells. At the time we were regularly checking our blood pressure readings using a home testing kit (I know it's not as reliable as going to your GP but it is an indication). He checked his bp, which is normally text book perfect, and found it was much higher than normal. Taking St J W was the only thing he had been doing differently so he stopped and a couple of weeks later his bp was back to normal.
Following this I checked out a couple of books in the library and found that raised blood pressure is a possible side effect and that you should monitor it whilst taking St John's Wort. The books I consulted said that this doesn't affect everyone, though. (Sorry, it was few years ago and I'm afraid I can't remember the titles.)
I'd also certainly check out what Faith has said about possible interactions with birth control if that is relevant to you. At the time I was taking St J W I was on the progesterone-only pill so I probably had a couple of close calls without realising it! I had started taking St J W on a friend's recommendation without checking for side effects - I have to admit, somewhat shamefacedly, that at that time I believed if something was herbal it was OK - ouch!
Having said all that I would still recommend trying it once you have checked out the birth control and bp issues if you feel they are relevant to you and are happy that you are not at risk.
Another thing that can help with mild depression is to take a good quality vitamin B6 supplement - I tried this after my husband's bp experience whilst taking St J W and found it very helpful.
Good luck!