How long does it take on average to receive one confirmation? Is it still reversible? Hi! Based on your experience, how long does it take to receive one confirmation for any given transaction which has payed the standard miner's fee? Is it still reversible?

Thanks!
A Bitcoin transaction, by design, will get one confirmation after an average of 10 minutes. Even before a confirmation has been received, a transaction is generally irreversible. 

If you were to send a second (double-spend) transaction, using the same inputs as a transaction you've previously sent, I suppose there might be some custom-developed nodes that would give it priority if the transaction fee was higher than the first transaction. However, I believe most nodes would reject the second transaction.

If one of these (corrupt?) custom-developed nodes was to solve a block including the second transaction, my understanding is that the network would accept the block and thus accept the second transaction. This question has been asked separately, here: Does any pool accept higher-fee transactions of a double spend, instead of the earlier one?

However, it's worth mentioning that standard nodes will not relay the second transaction.