I don't have a multiple entry visa, as a Brit I am visa exempt.
I sent my extension request in recently, I paid online, rather than applied online, because I wanted to attach quite a few documents.
Does the online application system allow you to attach scans of documents?
I know my paperwork is in Vegreville, they signed for it when fed-ex delivered my package, so my implied status starts in March.
Like I said, "Assuming they have received your extension application, you will have "implied status" as soon as your current visa expires"
So when the visa in your passport expires you have implied status and can stay until at least the point where a decision is made.
There is no way of telling if an extension will go through, the best anyone can do is submit a truthful request and provide everything they ask for.
Times for processing of extensions are here.
http://www.cic.gc.ca/english/information/times/canada/process-in.asp#temp_res