From looking at MB21's TX Gallery, most of the UK DTT transmission sites that appear to have Satback at the ready should their main feeds fail seem to be the around 50 so "main" sites where the fibre feed to the site could fail. Satback provides the backup for the PSB1&3 multiplexes, while PSB2 appears to rely on a different method (RBL?) or try to ride it out for the best! There's only one relay site I've seen (
Bruernish on the Isle of Barra in the Western Isles of Scotland) that appears to have Satback on standby presumably when across sea links fail, and there is/was a few relay sites of Bilsdale that had to resort to emergency Satback installations when its parent failed (appearing to use Triax 80cm or 110cm dishes as standard?), but it would appear that there's almost certainly less than 100 sites that have a "permanent" Satback installation (as opposed to the higher numbers for the DAB networks), so how long dual illumination between IS901 & Telstar 12V goes on will depend on available labour to move dishes, weather conditions etc. as well as sending someone out on a ferry to the Isle of Barra! Nothing unusual about this, in countries where satellite feeds are used as a primary means of feeding terrestrial transmitters, different satellites are regularly jumped on to for various reasons e.g. the UK national DAB feeds used to be on 4.8E.