I left the notion of mainstream intentionally ambiguous. Because it's hard to define.
One can use different criteria.
1) A percentage of the amount of the total expenses on one year. But even 1% would be huge and would imply a very high price for bitcoin. However, according to the mentioned Gresham's Law, bitcoin will be hoarded and only rarely spent. It's like gold. Gold is part of mainstream, but people don't exchange it much, they prefer to keep it. On the main functions of money: mean of exchange, unit of account and reserve of value, I think the last will be bitcoin's main function. It won't be used much as unit of value, because of its volatility (many places that accept bitcoin, prefer to announce the prices in USDs). It won't be used a lot as mean of payment, because people will rarely spend it.
2) Accessibility: the fact that anyone can easily buy, sell and pay with bitcoin. Clearly, this is decisive. If we could exchange and spend bitcoins on the majority of ATM and retailers, we might be able to say that bitcoin is mainstream. But imagine that even in these conditions bitcoin kept being scarcely used, with small demand. It wouldn't be mainstream. Many physical businesses complain they never had customers paying with bitcoin. So, this is a necessary condition, but it isn't enough.
3) A percentage of people owning it. This seems to be a good criterion. But it isn't easy to establish a number: 10% seems enough, but not 1% or even 5%. 70 million users on the all world would be great, but not enough for talking about mainstream. I guess only at 10% we would start to see unfold the problems above mentioned. But this kind of projections is hard to make.
Think about Paypal (I hate it, but let's use it as example). Is Paypal mainstream? I don't think so. It had a volume of transactions of only 180 billion on 2013, about 150 million active registered accounts and in many countries you can use it as mean of payment only on a few places. So, it reached about 2% of world population.
Bitcoin had about 23 billion USDs in trade volume alone during the last 12 months (see
http://www.bitcoinity.org/markets/list?currency=ALL&span=6m, at current price) and maybe 1 or 2 million active users. It still has a long way to go.