5mb
In modern a/c the FMS/FMGC database knows all waypoints and usually the published hold axis and direction.
Therefore, if you send us to somewhere off route (eg re-route us into LHR via BIG rather than LAM to balance the stacks) it only a matter of entering the new route - say 10 seconds or so.
To then have to hold somewhere short of the inner hold point (which into LHR I normally pre-programme!) it is just again of entering or selecting the waypoint, and selecting the database hold/modifying as needed for time outbound and getting colleague to crosscheck, of course!
Given 30 seconds notice of the hold it shouldn't be a problem, if you subsequently cancel the hold before the turn starts, again no problem, just select heading mode and then reprogramme the box.
HTH