Magnis would never commit to anything. But we do know industry norms. Looks like 4-6 weeks for a un38.3.
It's a commodity cert. Everyone gets it. Well almost everyone.

BATTERY PACK CERTIFICATIONS
What They Are, What They Cost, How Long Do They Take?

Lithium battery safety is at the top of the list for anyone who manufactures custom battery packs. The battery pack certifications listed here are near universal standard industry practice for leading companies in the electronic industry. Product safety is important to all product stakeholders and passing safety certifications are an independent means of assuring products are safe. The practice of testing to UL, IEC, and IEEC is routine for companies with years of experience and with in-house resources to manage the process.

Below you will find a chart that outlines some general guidelines for the costs and timing of these certifications.

		CE	IEC	UL	UN38.3
Packs Required	0	33 sealed, 11 open	52	16
Est Cost	$500 - $1,000	$6,000 – 10,000	$15,000 – 20,000	$5,000 - 7,000
Test Timing	4 - 6 weeks	6 - 8 weeks	10 - 12 weeks	4 – 6 weeks

https://www.epectec.com/batteries/battery-pack-certifications.html