Given that
shield lasts one round and uses up a spell every time, the bladesinger can do this at most 3 times (having already cast
Mage Armor) or perhaps 5 times (due to Arcane Recovery) before dipping into their 2nd level spell slots on any given adventuring day. If that's how the player wishes to play their character.
So, perhaps be sure to set up a combat no and then that has mobs so you can use mob rules (DMG p250). Or once in a while smarter enemies might notice the
shield go up once or twice in a combat and then might try to do something that gives advantage on attacks against the bladesinger - knocking them prone, pack tactics, blinding them with sand to the eyes, etc.
Or, what
@TwoSix and
@Ovinomancer said.