Always difficult to predict how early to queue; Mika hasn't appeared in Manchester for some time so there might be a lot of people eager to see him. Also it's a Sunday so people wouldn't be at school/college/work, so more might be prepared to queue earlier. On the other hand, it's Manchester and April so it will probably be raining - that might put people off.
Also this is at O2 Apollo - O2 users may have a priority queue - if you are an O2 user it might be worth checking this with the venue.