Michael, use a newer Zipstream enabled camera. It should easily give you twice the storage duration (and probably more) of the M3004 which is quite old and relatively poor quality anyway. As for the M3004 storage on 64GB, you might get around a week, though it depends of now busy the scene is, etc.
See Testing Axis Zipstream and Axis WDR Zipstream Low-Cost M1125 Tested. Zipstream enabled camera list here.
I'd agree with using a newer camera if possible. The M3004's bitrates run high, often up to 2-3 Mb/s even in decent lighting. They also get very noisy fast when lights drop.
I'd also agree with the assessment of "maybe a week." That sounds about right based on the bitrates I've seen, plus the activity levels I'd expect there.
As a rough rule of thumb, every 100Kbps of bitrate will use approximately 1.2GB of data per 24 hours.
Your mall is open 9 hours a day 3 days a week, 12 hours a day 2 days a week and 7.5 hours 1 day, or roughly 58.5 hours per week.
For every 100Kbps of encoding rate you would use about 3GB of storage per week.
A typical 720p camera would probably be around 1.5-2Mbps for an average mall scene. This would give you 45-60GB/week of data if the cameras were recording full time during the hours the mall is open.
Lots of variables here, but a 64GB card sounds about right, assuming that 7-9 days of retention time is sufficient (assuming motion detection recording can cut that 45-60GB/wk down a bit).
Agree with Ethan and John about changing the model. This is camera may not be a good choice for edge recording because of its higher than normal bit rate.
From the Bandwidth Guide For Video Surveillance, this panel is relevant:
Mind you this is a well-lit static scene, and both are Axis cameras set at 720p and only 10 FPS with the same Q compression value (28). Yet the 3004 has nearly 3x the bitrate.