Hi detection26.
Hard to answer as a lot depends on the actual metal, the shape, the size, the texture of the surface, the orientation of the object in the ground, etc.
But to give you a guide.
I've dug a lovely big bolt at 1.2 metres (measured). Faint signal on the surface and very disappointed that it was a bolt!
I've heard of a few very large nuggets dug over a metre deep.
Have dug .22 projectiles 30cm deep.
Have dug LOTS of rubbish targets over 50 cm.
But on my GPZ training day there were 2 blokes from Persia (I think it was) that had bought the GPZ to take back to Persia for treasure detecting. The trainer told them they may have just wasted $10,000 on their new detector!! The reason being is that the GPZ has no discrimination and no depth advisory at all so you will be digging every piece of metal from Calais to Foix.
Gaining ultimate depth may not end up being as important as gaining some efficiency in how many holes you dig by having a discriminating detector.
Maybe a CTX 3030 might be the go?
Whatever you decide please make sure you come back here and post your treasure finds