How long after first contact, where editor says "manuscript is still under review", is it okay to enquire again about status of manuscript?
As an editor of a number of mathematics journals, I would be pretty irritated to receive two queries from an author within two months. What exactly is achieved by emailing a journal to ask for an update in the first place? Mostly, it helps on the (perhaps not so rare) occasion where the paper has fallen through the cracks, as happens when someone agrees to review the paper but then forgets about it. What would be achieved by emailing the journal twice in two months? It's hard to imagine it resulting in your paper being reviewed any sooner, and easy to imagine it annoying the editor. I would wait six months between queries.