Yes, your math is off by 37 orders of magnitude. You’re essentially calculating probabilities of independent low-probability events – create one improbable antibody this week, create another next week. The probability of creating both is the product of the individual probabilities, which is equivalent to adding the logs of the probabilities. So you add the number of bits. Generating 60 bits per week, and assuming independence, means that it will take a little over 8 weeks to generate 500 bits.