Throughput is 1000 transactions per second. If the network load was demanding 2000 transactions in a second then it would take 2 seconds to fulfill that need. If that need was maintained for 4 hours, then how long does it take the last transaction at the 4th hour to process?
I think this is the type of question this is asking. I will try to answer:
At the end of the 4th hour we have been asked to process 8000 transactions. Of those, only 4000 were fulfilled. That means we need 4 more hours to fulfil the last transaction. Let us try to put this in a formula, but please note this formula is taking into account a constant need where in reality need fluctuates. We also need to know the current backlog size.
Trx Backlog Qty
4 hours * 60 mins * 60 secs * 1000 unfulfilled trxs = 14,400,000
Transaction Wait Time (Seconds)
Trx Backlog Qty (14.4M) / XLM Transactions Per Second (1000)
Transaction Wait Time (Seconds) = 14,400 seconds = 4 hours
One more example at x7 overload over 5 hours.
Trx Backlog Qty
5 hours * 60 mins * 60 secs * 6000 unfulfilled trxs = 108,000,000
Transaction Wait Time (Seconds)
108,000,000 / 1000 = 108,000 seconds = 30 Hours
It would take 30 hours for the last transaction at the 5th hour to process.
This is why when an ICO happens on ETH, people have to wait a long time for a transaction to process.
Peace!