My first answer to this question is: I sure hope not! But my second answer is a bit more thought out. Elizabeth May has now run in one federal election and one by-election, so it is useful to take a look at her decision to run in Saanich Gulf Islands next election and why she chose that riding.
In all honesty, she didn't have much of a choice. The other option, Bruce Grey Owen Sound, was already occupied by a 'star' candidate who had very strong local backing as a result of his employment by Bruce Power at the nuclear power plant there. Elizabeth May, running against nuclear power in a riding where people are employed at a nuclear power plant is not going to fly.
But, as my question asks, can she win in Saanich Gulf Islands?
Lets take a look at her previous attempts and compare them to how previous elections went. In London North Centre, she drew votes from pretty well every party, including the Conservatives. Comparing previous and later elections to the by-election results we can see this pretty clearly. However, this was a riding with no incumbent and it was a by-election which can sometimes bring out strange results. However, in recent years this conventional wisdom (that by-elections produce strange results) has been broken by the NDP victories in Montreal and Windsor-West vindicated by later federal elections and CPC victories in Saskatchewan and rural Quebec, also later vindicated in federal elections.
Figure 1: London North Centre Election Results
In Central Nova, running against a well known minister and with no Liberal candidate, we can see that the net result was for more Conservatives votes overall, but it can be inferred from the London by-election that some Liberals voted the Conservatives rather than voting for Elizabeth May and that some Conservative voters were lured away by Elizabeth May. The NDP in both cases, dropped by 7% and 13% from previous years in London North Centre and Central Nova respectively despite running decent candidates in both. They provide our most solid basis for evaluation.
Figure 2: Central Nova Election Results
So, we've learned a few interesting things.
1) Based on London North Centre and Central Nova results, Elizabeth May can increase her share of the vote by 7-13% from the NDP.
2) Based on the London North Centre results, Elizabeth May can increase her share of the vote by 5-8% from the CPC.
3) Based on the London North Centre results, Elizabeth May can increase her share of the vote by 5-6% from the LPC.
This looks quite hopeful as it means that potentially, her presence in a race could increase the Green party share of the vote by 17-27%.
Her next election in the Saanich Gulf Islands riding will look very similar to the one in the London North Centre since it can be expected that the Liberals and NDP will both field decent candidates to try to oust the Conservative incumbent. Looking at the results for that riding it can be seen that naively assuming the most optimistic scenario where the Conservative vote share drops 8% and she increases 27% she would take the riding by a hair (37.5-35.5%).
But unfortunately it isn't that simple since the NDP vote share in 2008 was artificially low because the NDP candidate dropped out late in the election. Thus, using the 2006 results is more reasonable and results in pretty much the same result (37% - 32%) assuming the most optimistic scenario for Elizabeth May.
Figure 3: Saanich Gulf Islands Results
So, it is possible for Elizabeth May to win assuming that she can draw votes from the Conservatives, Liberals in the same proportion as she did in London by-election and from the NDP in the same proportion as she did in the Central Nova race. Still, its a long shot. Eve