You can’t know that until you try it. You could always pick one of the smaller scripts, work through it methodically until it’s fully 7-compliant, then review how much work it was and how long it took you, remembering that the first time will involve more research and you’ll get quicker as you go on.
It would just be a guess. You’re only guessing how many files there are, but we’ve no idea of whether that’s an accurate number or not, and it doesn’t really matter - what matters is how many lines of code need to be reviewed, and how badly-written (or highly optimised, either can make it harder to read) the code is.
Where has the script come from? Do they have an updated version, and if you’re hoping to use it as part of a site that you might make income from, are there any restrictions on it?