I agree, and that wound looks pretty clean(not slimy/snotty) so that will help your odds if you can keep it like that. Syphon out any infection if it occurs.
I'd frag that acro a little past its receding tissue to as to help avoid any infection from spreading too.
If the echino doesnt receed anymore Id speculate itd be about 4-5 weeks before it covers over completely, 6-8 for it to be virtually unnoticable.
-Justin