(coincidentally you'll find most Apch plates annotate "lead radials" which are 7 before!)
Pans Ops recommends 2NM of lead for a turn off an arc, regardless of the arc radius, and I'd be most surprised if TERPs is any different to that. Thus, the lead radial will differ according to the different arc radii that you might encounter.
If you only ever encounter a 15 DME arc, the 7 radials will be about right, but bear in mind that Pans Ops sets a minimum arc radius of 7 NM. I've seen many approach charts that use a 10 DME arc, so the 7 radials of lead will be too much.