How do you punctuate something you paraphrased?

There are two common methods for handling alterations to a quote, depending on the nature of the alteration.

For omissions, the most common practice is to insert an ellipsis . . . in place of the omitted words or phrase. For alterations, such as to preserve grammatical flow in the context of a paragraph, to work around an omission, or to add explanatory or clarifying text, placing the new or changed language [in square brackets] is the standard.

That said, in the context of a tweet, if you're talking about committing a grammatical sin on somebody's behalf to fit within a 140 character limit, as in your example, I'm inclined to simply say: relax. It's not that big a deal, and changing 'within' to 'in' is hardly misquoting someone to the degree that it requires any consultation of a style guide.

There is no generally-recognised punctuation to mean "this is a close but not exact quote". The standard way of indicating this is...

To paraphrase Churchill, never have so many paid so little for so much.

...which clearly won't help OP, since it would take more characters to indicate what he's doing than would be saved.

There is a convention that you can replace part of a quote by some semantically-equivalent text in [square brackets] - for various reasons, including space-saving. So in this case OP could write...

"If I couldn’t do it [quickly], I couldn’t do it at all".

Ditto LessPop_MoreFizz.

In your example, you could write "If I couldn’t do it [in] 10 min, I couldn’t do it at all."

It's common to add very brief elaborations in brackets, like "Before being elected president, he [Theodore Roosevelt] served as", etc.

You can always reword broadly just by removing the quotes. Instead of writing

Bob said, "If I couldn’t do it within 10 min, I couldn’t do it at all."

You could write

Bob said that if he couldn't do it in 10 min he couldn't do it at all.

Then you're giving proper credit without indicating that it's an exact quote.

Frankly, the example you give is not the best. You're saving 4 characters. Any punctuation or explanation you give will likely take at least a couple of characters, so the savings ends up being minimal. I suppose in a tweet the size limit is so tight that you're in a bind. Conventional English rules for handling quotations and variations thereof really just weren't invented to work within that limitation. Maybe we'll have to invent something.

I'm surprised at all the hoop-de-doo over a simple and clear question: wanting to paraphrase a quote, however, the in example, "if I couldn’t do it within 10 minutes, I couldn’t do it at all", to simply shorten "minutes" to "min", which is the product of a technological limitation which exceeds the bounds and rules of humanely generated grammar to which you can artificially attempt to adapt to computerese, in which case, any grammatical application that approximates your exact meaning is probably okay; however, the above example is not a good one. To use someone's quote, but express it in a shortened way or by using your own words or interpretation of the quoter's intended meaning, I use a single quotation mark [an apostrophe on your keyboard] to do this. For example, using a Bob Marley quote,"The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively". You can paraphrase by writing: I agree with Bob Marley who sings about,'greatness shouldn't be measured by one's wealth, but by how one affects those around him'. Hope this post helps.

I found these two the most direct and clear answers.
+++ Two common methods for handling omissions and alterations to a quote.

For omissions, the most common practice is to insert an ellipsis . . . in place of the omitted words or phrase. For additions, placing the new or changed language [in square brackets] is the standard. +++ To use someone's quote, but express it in a shortened way or by using your own words or interpretation of the quoter's intended meaning, I use a single quotation mark [an apostrophe on your keyboard] to do this. For example, using a Bob Marley quote, "The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively". You can paraphrase by writing: I agree with Bob Marley who sings about,'greatness shouldn't be measured by one's wealth, but by how one affects those around him'.

Don't know if there's a convention—because before Twitter, the 3-point ellipsis (. . .) + [brackets] served everyone's needs very well. So in my opinion, John McIntyre's instinctual solution of 2 "similarity" signs (~~) seems like a good one, although a single "sim" sign (~) saves space, so in Twittereze might be just as good. As I think on it, though, the "approximation" sign (≈) might be better.