Is there a fixed amount of time an employer has to provide you with your ROE? If so, what is it?

Depends , here you go
Chapter 1: Understanding the Record of Employment Form - Employment Insurance - Service Canada

So 5 days for me - Thanks!

In reality it does not always happen that way. If you are filing for EI, you can still make a claim without and ROE and EI will contact the employer if they don't send you one.

I've never recieved one when I've left a job.

Not issuing an ROE is an offence with penalties up to $2000 and 6 months in jail. One should be given to all employees, whether they claim EI or not.

Within the timeframe?

I didn't get one from Sears, The Brick, Eddie Bauer or Country Cupboard, the only one I got was from Zellers, I was there a whole 9 days, which was 7 more than The Brick

They may have been filed electronically straight to Employment Canada.

You may not get a physical ROE anymore depending on employer, some issue them electronically to Service Canada.

Other then small companies with few employees, all my employers filed electronically in the last few years and I never actually received one in the mail.