Staking Polkadot with Ledger

Yes, it is dynamic amount of around 40 DOT (the actual number can vary). It can move up or down depending on the nominations and the election solutions. Nominators with a stake smaller than this amount may not currently be receiving staking rewards.

An amount of rewards is available for each era (every 24 hours). To potentially receive rewards, you will have to wait for the end of the next era. For instance, if you nominate a validator during era N, your nomination will go into effect era N+1, then payouts are possible when era N+1 ends and era N+2 begins.

The global amount of rewards available for each era is shared evenly between validators, regardless of their stake (the payout of a specific validator, however, may differ based on era points collected). Each validator then applies a commission. For each validator, remaining rewards are then split among nominators for which the validator was active for this era, proportionally to their stake. For each validator, only the 128 nominators with the largest stake will receive rewards.

Rewards payment can be triggered by any user (usually the validator or one of his nominators). When any of them triggers the payout, all of them receive their rewards. Claiming rewards manually is not supported in Ledger Live. As a consequence, you will receive your rewards without doing anything when the validator(s) you choose or anyone else who nominated the same validator(s) triggers the rewards payout.

With Polkadot the validator and nominator could lose a portion of their assets if the validator misbehaves and is slashed. As such, it is important to take a good look into who you are nominating.