This question is a companion to this one that focused on crafting an improvised spellcasting focus. Here I wish to focus on using such an improvised focus.
If a caster typically uses a spellcasting focus to cast spells and loses it, and they are in a dungeon or other location where they cannot purchase a new one (or even purchase special materials for a new one), and they try to craft a makeshift focus, how do we determine if it's "good enough" to work? And can they use it immediately, or would they need to spend some time getting used to it?
A couple of drudic foci (sprig of mistletoe or totem) are available for purchase for 1gp. Arcane foci and holy symbols start at 5gp. This suggests a certain level of craftsmanship and/or quality materials. Thus is seems that a wizard can't just pick up any old wooden branch, peel off the bark, and declare that wand-like length of wood to be his new arcane focus.
For an improvised focus crafted perhaps imperfectly by someone without access to the "proper" artisan tools, is there anything in the rules to guide a DM in deciding how well it would have to turn out to be effective as a spellcasting focus?
I also don't see any rule that says a caster needs to spend time practicing with her new focus -- have I missed something?
I'm interested in any rules references people can provide, as well as any playtested homebrew rules people have used for this sort of thing.