According to the solana docs
Alternatively, an account can be made entirely exempt from rent collection by depositing at least 2 years worth of rent.
Does this mean, that so long as you deposit 2 years worth of rent, your account can never be purged?
Yes correct, when you deposit more than 2 years of rent, your account is rent exempted. Actually most projects are using the rent exemption amount by default. There are not a lot of usecases where you would want to deposit less than the rent exemption amount since you can get your SOL back when you don't need the account anymore.