It partly depends on the variety you have and how much it spreads, but I've seen 2' apart recommended for forsythia hedges (for plants that reach 8' in height). That's if you're going to keep it trimmed like a more formal hedge, which will also reduce the number of blooms you see in the spring. If you want a more natural, untrimmed hedge, and more bloom, I'd plant them slightly less than the mature width apart (ie, if they have a mature spread of 4 feet, I'd plant them about 3.5' apart). Happy gardening!