Wirex is part of the BitGo Instant network. This means you can send bitcoins to Wirex accounts from another Wirex account instantly (off-chain). This instant transfers feature will also work for funds transfers to another bitcoin wallet that is part of the BitGo Instant network.
For bitcoins sent from Wirex account to other third-parties outside of BitGo Instant network (on-chain), the time will depend on how many confirmations the receiving side requires. Confirmations are an essential part of how the blockchain transfer works. You can find more information about Bitcoin confirmations in this Wiki article.