Sometimes, when downloading files, the download will stop with the text "Transfer Timed Out" below "Receiving File", how do I fix this?

Simplest solution: Don't download, per the sticky located here

Next: Check your DCC timeout settings and ensure that they aren't set really low. I generally find 120 (2 minutes) is plenty for pictures, text files, etc.

Who said they were illegal files?

Anyway, I've set my time-out for 999 but I doubt large files will download within 8 minutes, any way to turn it off?

That's not what timeout means, it times out after not receiving a packet of data from the source for 999 seconds. Unless it's happening with every source, the problem is most likely on the senders end. Try to download a large file from someone else.

"Large" files are rarely anything but illegal. There are few legal files that are large. In any case, the fact that 99% of downloads done in mIRC *are* illegal means that we don't help anyone with DCCs other than very basic information per that sticky. That's what other forums and channels are for.

Anyhow, if your download is timing out, there is little you can really do about it. It will probably be your ISP that's causing loss of connections. The best you could do is switch to another download method that does not need a semi-constant download stream to keep downloading, or else find a better ISP. Note that, as mentioned, this could also be the sender's ISP problem.