NSString has a sizeWithAttributes: method that can be used for this. It returns a CGSize structure, so you could do something similar to the following to find the width of the text inside your label.
iOS 7 and higher
CGSize textSize = [[label text] sizeWithAttributes:@{NSFontAttributeName:[label font]}];
CGFloat strikeWidth = textSize.width;
iOS <7
Prior to iOS7, you had to use the sizeWithFont: method.
CGSize textSize = [[label text] sizeWithFont:[label font]];
CGFloat strikeWidth = textSize.width;
UILabel has a font property that you can use to dynamically get the font details for your label as i’m doing above.