I was prescribed 15mg of prednisone for my scalp and skin inflammation/irritation for two weeks. how long until i feel relief and is this a high dose?

Not a high dose but.: You do not describe the type of inflammation/irritation you are experiencing, but generic xantac (ranitidine) 150mg two times a day and Claritin (loratadine) 10mg will give you better relief if itching is an issue!

Prednisone: Prednisone is a steroid medication. Oral preparations of steroids will take about 12-24 hours to feel clinical relief. 15 mg is not considered a high dose. In my opinion, a high dose would be over 60 mg a day.

Topical: You can probably do fine with a strong topical steroid scalp lotion instead of taking prednisone. Even more important, find out what may be causing it by seeing a dermatologist else the same problem will come back. Prednisone takes 6 hrs to start working and the peak effect may not show until 24 later at least on the skin. I would use it only as a last resort.