Eh, an amphibious invasion is hard, but I also doubt it would be nearly so one sided or bloodless as some seem to be suggesting.
The Chinese on their own have a fairly high level of ambitious landing, and the range from mainland to Taiwan is still only about 200-300 km, so helicopters can drop light infantry, well, anywhere on the Island if needed be on top of all the amphibious landing capacity, which looks like it might practically double in the next 2-3 years.
My leaning is that China will invade when they reach the point when all the fighting they have to do amounts to a helicopter flying across and landing in front of their parliament to inform them of the conquering with barely a shot fired. Basically, once the will to resist is well broken and infiltration near complete. Basically, they want to take Taiwan with as much fighting as the Taliban needed to do in the last week to take Afghanistan.
If the Tiawanese don't immediately fold under pressure, then I think it comes down to a 1-2 month air and naval war where the US and Tiawanese commitment is tested: after the 1st or 2nd Aircraft Carrier is destroyed, will the US Navy keep to the field, or leave? At which point the Taiwanese may read the writing on the wall and surrender, or the bombings and air casualties will just be intolerable, and they give up that way.
If an intense 1-2 months of air and naval campaign doesn't break someone's naval or air capacity, and its not going to come down to a morale victory, then its probably a year or two campaign to take the Island: Iwo Jima was a month long operation over a tiny space, the Philippine campaign was about a year long, Okinawa was like 3 months. But, if people are committed enough to keep up a multi month war to a yearish long, Taiwan at that point is probably not the all important front at that point, and you probably have an invasion of Korea to draw the US into massed ground campaign or secure more airbases and cut off some more support harbors for the US Navy or Chinese occupation of Singapore down to Australia or some such to secure oil and cut off Japan.