Making this bit mor concrete: luks1 uses pbkdf2 with sha1 and minimum iteration count of 1000 as far as I can tell. Looking at random hashcat benchmark for rtx 4090, the closest thing is about 20MH/s. If we assume attacker spends a month with 1000 gpus, they will have bruteforced 20e6×86400×30×1000 combinations. Log2 of that rounds up to 56 bits.
You're underestimating the default PBKDF parameters used by LUKS1, `cryptsetup luksFormat` [1] benchmarks the `--pbkdf` function at runtime when configuring the keyslot, and the default iteration count is based on a target `--iter-time` [2], which is either 1s or 2s [3], unless overridden by the distro. I'm not entirely sure about the PRF (SHA1/256) used by LUKS1, it might be SHA-1 or SHA-256 [4].
For a LUKS1 image created with Ubuntu 18.04 on a i5-8265U ( Q3'18) CPU, that appears to result in an iteration count on the order of 1-2M. Assuming those hashcat benchmark numbers scale linearly on the iteration count, you're down to about 10kH/s for PBKDF2-HMAC-SHA1 or 5kH/s for PBKDF2-HMAC-SHA256.
Assuming 10kH/s, that drops you down to roughly 45 bits of entropy for 1k gpu-months, or 250k gpu-years to net 56 bits of entropy. Those would be on the order of [a-z0-9]{8} (41 bits), [a-zA-Z0-9]{8} (47 bits) or [a-z]{12} (56 bits).
OTOH it's easy to generate a 20-character password with far less entropy.
https://gist.github.com/Chick3nman/32e662a5bb63bc4f51b847bb4...