Every node has its own mempool and the purging rate also varies according to that, the purge value we see on explorers are for the default size but as far as I see, ViaBTC includes the TXs above 23sat/vbyte in their blocks constantly so I assume that could be their purge rate or I may be wrong but ViaBtc always includes the submitted TXs in their next blocks or atleast in the two and if it's taking more than that then the TX may not be submitted for acceleration.
If you noticed on ViaBTC that the 100 submission in an hour is not used up very fast and taking anny minutes or available all through the hour, that is when this happens, in a way it may take 10 hours or more before ViaBTC will later include the transaction into a block after many have been mined by the mining pool but not including the transaction as it is on a queue. To only thing you can do to still know that your transaction will likely still be accelerated if not mined in the next block mined by ViaBTC in this case is to what what I have posted.