You will immediately see it in Electrum's history the moment that it's relayed to the mempool of the server where you're connected to.
In simpler terms, it should show-up a few seconds after the sender broadcasted the transaction.

Now, if it's not showing as unconfirmed or confirmed after a few minutes, check your connection icon (circle) on the upper-right if it's connected to any server. ("app" = android version?)
If connected, check if the sender already sent the transaction (some exchanges send in batches if it's from an exchange).