Will every message x reach all connected peers in time t?
Can we take for granted that if in time period t there were 10,000 connected peers (no new peers connecting, no peers disconnecting), all with good internet connection and hardware, then if every of those 10,000 peers originated one transaction (or any kind of message) to his peers, can we take for absolutely granted that eventually every single one of those 10,000 connected peers have in his mempool all 10,000 transactions, including his own and those of 9,999 other peers and absolutely no transaction will be missing in mempool of any of those nodes (they will all have same set of data)?


If you also include that no one is acting maliciously, then yes, under these perfect conditions with no malicious actors, you take for granted that a transaction will reach all nodes within some unknown time t. However, if there are malicious actors, then no.


  If yes, and would could be estimated time (seconds, minutes, hours)?


This is impossible to know. It depends on the hardware, what the transactions consist of, the internet connection, etc. At best, it would probably take a few hundred milliseconds. At worst, likely a few minutes. Without more specific parameters, it is impossible to know.