How can I figure out how long is a reasonable time to take on a task (especially with Meta' s move fast culture)?

Your tech lead/manager might be being nice to you as you're new and don't want to put pressure on you by assigning due dates to tasks. However, even newbies are being judged at Meta, and you can't take forever on tasks. Have that awkward conversation with your tech lead/manager and ask them for due dates, effectively giving them the permission to put that pressure on you (which also adds clarity, which is critical at Meta).

I recommend asking for P80/P50/P20 estimates as well, which is how Meta does timelines:

• P50 - Standard Meta goal. Ambitious but possible (50% chance of hitting)
• P80 - 80% chance of hitting, safe target
• P20 - Incredibly aggressive, 20% chance of hitting. If you hit it, you are a rockstar

And how do you balance speed with learning?

I recommend this: "How to avoid going down the rabbit holes when learning new things?"

If you are doing things properly, you should have both. Striving for quality brings both speed and learning when done in the right balance.