how long can I delay sending full?

Why not send them an email explaining what you are doing and that you will send the full in a week? I don't think it's a problem but I would communicate that to them.

I've seen this question posed here before (in one of the agent threads) the response was that within a week, they don't really care. If you're going to be longer than a week, just write them a note explaining the situation. They're fine with it, they'd much rather have the new-and-improved version than waste their time with your old crappy prose.

I concur with the above. A week or less, just send it, don't clutter the agent's inbox. I regularly have fulls trickle in after I empty the query box.

More than a week or two, and an explanation may be in order.

On the other hand, I've seen writers on (other) forums say they need more time to 'fix things up,' or 'finish a chapter.'

I tend to think, what the heck? You're querying even before you're finished?

Yes, and I've seen this more than once.

I think this is a tricky subject... honestly IMO if you send a note saying you need to fix this one thing, or finish this chapter, or your not as ready as you thought you were, or whatever the agent is going to be leery, because you've just broken like the cardinal rule when it comes to query... you queried a novel that wasn't ready.

Some have a problem with it, but most realize that authors are human and work on a learning curve. Every time I look through an older manuscript I find tons of things I want to change, just because I'm improving in my craft. Agents realize that, and most of them are pretty forgiving.

However, I have come across one who was quite rude to me when I wanted to send an updated full to her. I won't mention specific names, but let me say that her attitude does match her name.