In terms of how far it’s going to set you back on your current level, it depends on the item. If you’re reviewing an item from another level that’s already Guru or above, it won’t set you back at all (just getting too many wrong too often will eventually make your review count go up). If it’s a kanji or radical from your current level, it’ll set you back a certain amount of SRS stages (It might be 3, I’m honestly not sure. 2-4 I’d bet.) There are a few stages within Apprentice, so It won’t make it like a new item again if you’re on Apprentice 4.
With total time on level, some items are much more important than others. Make an effort to review your radicals as soon as they come up and get as few wrong as possible to unlock the rest of the kanji. It seems like a lot, but it’ll reduce your time on level for hours, if not days. Meanwhile, the kanji can be wrong several times without affecting your time on level at all (as long as you can finish them before you finish the new kanji). You can speedrun reviews, just be sure to regognize the “important” items from your current level. Those are the radicals and whichever set of kanji are on the lowest SRS stage. With those, every hour you put off reviews is an hour added to your total, and every time you get one wrong you’re setting yourself back. (You have 2 or 3 kanji “grace” each level though).
Also, download an answer override script. Sometimes I hit enter twice and miss my chance to override so the problem still exists but it’s a huge help with typos and synonyms.