Large-scale tree planting can remove some CO2 from the atmosphere, but nowhere near as much as humans add by extracting and burning fossil fuels. See https://skepticalscience.com/1-trillion-trees-impact.html for a detailed assessment of what this looks like.
The IPCC has a chart showing what actions need to be taken over the next few years. Afforestation is one piece of many things, all of which we need to do.
I think many Republicans have always had the opinion of “we will just fix it when or if it becomes a problem”, despite the fact that most studies (and common sense) has shown that it will be harder and harder (and more expensive) the longer we wait, or possibly not fixable at all.
So to answer your question: No, they have no fucking clue how long it will take or how much it will cost to plant 1 trillion trees.