If using the [c] flag and you want to interrupt it, a convenient way of continuing where you left off (and not touching previous lines, which may have new matches, depending on the nature of the replacement) may be :.,$&&<Enter>, which means “operating on the range current line (.) to end of file ($), execute the last substitution (&) with the same flags as last time (&)”.
(This is very unlikely to interest you, but I mention it for the sake of others. I’ve commonly seen people not really know about ranges especially, never stopping and thinking about what the % means (see `:help :%` if you don’t know), beyond possibly having encountered '<,'> by accident when pressing : in Visual mode (for explanation of these, see the help for :', '< and '>).)
(This is very unlikely to interest you, but I mention it for the sake of others. I’ve commonly seen people not really know about ranges especially, never stopping and thinking about what the % means (see `:help :%` if you don’t know), beyond possibly having encountered '<,'> by accident when pressing : in Visual mode (for explanation of these, see the help for :', '< and '>).)