What do you mean? The mempool is never
at X sat. It's just a pool of unconfirmed transactions.
When a new transaction is received by a node, it will hold it in its memory pool with all the other latest transactions it has received. From here the transaction will be hoping to get selected for inclusion in the candidate block.
So basically, the memory pool is a waiting room for new transactions.

You are probably referring to the currently recommended fee. Everyone can choose how they recommend it, but most sites recommend a fee that is high enough to get a transaction confirmed in the next block. It depends on how 'filled' the mempool is, and this depends on how many transactions were submitted lately.
On average, the number of transactions that can be processed stays constant, so if the number of new tx's is higher than that, the pool fills up and if it drops below, it is cleared block by block. When it's empty or almost empty, you can get a transaction confirmed in the next block for 1 sat. But nobody can tell you or guarantee you when the mempool gets this empty.
If you want to observe it better, I would recommend using
https://jochen-hoenicke.de/queue/#BTC,8h,weight and
https://mempool.space/.