How long does it take on average to receive one confirmation? Is it still reversible?
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.