Testor's Dullcote is awesome because it has a very matt finish and it very rarely has any problems during application, I use it all the time as a last layer on miniatures, but due to its expense, for things like terrain I use other matt varnishes.
You absolutely do not need to wait 24 hours between applications of Testor's Dullcote...I have been using this stuff for years and years and on a good day, you can put a second coat on after 30 minutes, no problems...if it is cold and humid, I'd wait a few hours though. Make your coats light too, don't layer it on thick. Better to do 2 x light coats than 1 x thick coat.
For protection, Testor's Dullcote is only average, it certainly does not offer the best protection out of all the varnishes you could use.
For metal miniatures, you need to have a really good primer/undercoat...I always use a non-hobby orientated primer for metal, something that you use for automotive painting is good, I find they give a good solid prime on metal miniatures...paint as normal, then choose a really good gloss varnish. Gloss dries harder and is more durable than satin or matt...once the gloss varnish is dried, give it a couple of very light coats of Testor's Dullcote to remove the shine...and if you miss any parts like under arms where the spray didn't reach, cover that with a brush and matt varnish.
The Army Painter dips give an excellent finish in terms of durability for metal miniatures...if you found a suitable product that is similar, but in clear, it would make an excellent finish to prevent chipping and rubbing off of paint on metal miniatures...follow it by light coats of Testor's Dullcote to remove the shine.