That is a certainty. The only question is when. Will it be 2025? 2035? 2049?
With China's phenomenal progress in military technology and overall comprehensive national strength, I cannot see Taiwan remaining de facto independent past 2049.
By the 100th anniversary of the founding of New China, I'm willing to predict that Beijing's patience will have been exhausted.
Within 15 years, likely. I will attempt to quantitatively explain. because "living standards" is a very broad term that does not take into account public conveniences (after all, a country with no military, no public transportation, no roads, but everyone live in mansion and have private jet is a high living standard but useless country) I can only use a more "fuzzy" term: Psychological equality. That is when the notion that two people are "in the same economic category" is applied, that there is no significant discrepency in wealth and two people can talk as equals.
According to what people consider a "developed nation" we can use the US as an example, and consider that US's GDP/capita is 3x that of taiwan which is 4x that of China according to IMF 2010, and considered "developed" with "comparable salaries". So we can say that countries whose GDPs are within a factor of 3 of each other, are "equals".
This also explains why Chinese cannot see Indians as equals: Our GDP/capitas are not within a factor of 3 of each other (4300/1100 > 3). We are not in the same category - we are a developing-developed country while India is a developing-undeveloped country, since we are not within a factor of 3 of each other. This gap is also not closing, but rather widening, further increasing the gulf between Chinese and Indian psychologies.
(
List of countries by GDP (nominal) per capita - Wikipedia, the free encyclopedia)
At current growth rates, it will take only 1 year for the ratio of US:Taiwan GDP/capita to match Taiwan:Mainland GDP/capita. Essentially, we are already "in the same category".
For total integration, US:UK GDP/capita ratio is 4/3. Assuming constant 2009 growth rates of 9.1 for mainland, and 2% growth average for Taiwan (not counting the 2009 negative growth), the time needed for Taiwan/Mainland GDP ratio to reach this value is 2023 at 21,000:15,000 USD.
This is the upper bound on reunification; it can be adjusted downwards by appreciation of the RMB but is unlikely to be adjusted downwards in the light of mainland's previous economic performance. The lower bound, of course, is not limited by anything other than political pressure by far right radicals like Chen Shui Bian and Li Deng Hui.