Her question could have meant how long was it going on before this or how long will it be before the matter is put to bed, and Oz wasn't sure how to answer right away. He knew the answer to the first question would win him no points with his wife, because his affair with Sophia had been entirely too long, and he'd known that — probably Sophia had, too. But he couldn't answer the second question with even the slightest degree of honesty, because he didn't actually know that it was going to be handled, and could not even definitively say that he'd made any progress in that direction tonight. The whole scene at the ballet might have done more harm than good, depending on how Soph had read it... and after how unpredictable she'd been in their last argument, he couldn't pretend to guess her mind on the subject. She was being irrational, emotional — as women often were, he had heard, during pregnancies. He would attribute it to that, because the alternative was admitting that he had never known her quite so well as he had believed, and even now — even when things were almost certainly over between them — the idea still chafed.
"I barely saw her at all during the campaign," he said, which was sidestepping Thomasina's direct question but trying to answer what he supposed might have been at the heart of it. He wanted to convey that he had not been avoiding Thomasina at any point in order to run off to the ballet — though in fact that had happened once, the last time he'd seen Soph — and again tonight, technically — but it was hardly a habit. When he and Sophia were not in the midst of crisis, he didn't prioritize her over Thomasina, and he wanted his wife to understand that — that if he had neglected her in the past few weeks or months, it was because he was genuinely busy, not because he had pretended to be busy and then gone off to seduce Sophia.
"I've never bought her rubies," he offered (rubies being his Christmas gift to Thomasina, last year) — he did not volunteer that he had been more than willing to shower her in expensive gifts and had only refrained because she had seemed offended at the idea of her affections being purchased, nor that he had sent along chocolates for her children last Christmas (this on its own did not seem particularly incriminating to him; he also sent chocolates to Locke's children, and Sina was hardly jealous of him). "I've never taken her to a dinner party or a dance. I've never been with her, as far as anyone in society knows."
In short: he'd played by all the rules he'd established for himself, when he married Thomasina. He'd done a mental inventory of the things he thought were necessary to keep his promise to her — to keep from embarrassing her the way Locke would have embarrassed his future wife, while he was openly cavorting with his mistress and keeping her in a flat in Pennyworth and helping raise her children. All the rules except the most important one — he'd gotten her pregnant, and if he couldn't resolve that without Thomasina finding out about it she was probably never going to forgive him... but everything else, he thought, was forgivable — even if she was not in a particularly forgiving mood at the moment.
"I met her last summer," he finally admitted, to the actual question she had asked.

MJ is the light of my life <3