mathematics - How long after first contact, where editor says "manuscript is still under review", is it okay to enquire again about status of manuscript?

About a year ago I submitted a paper to a very good math journal. After 10 months I asked the editor for its status, and he said it is still with the referee, and that he will remind him about it.

Two more months have passed. Should/could I write the editor again? or am I risking annoying the editor/referee? How much time should I wait between such inquires?