I think this question is ill-posed.
Most of the time that I have seen the matter posed as a "break-even" decision, it is framed as "I need XX years of tax-free growth to make up for the taxes." But this is the wrong way to look at it. It really is about the tax rate when the money is withdrawn (either now, by conversion, or later, for spending).
To simplify, consider $1,000 in a tIRA. Further, let's assume that you will be in the same tax bracket (say, 22%) both now and later, and that you will pay for taxes out of the tIRA funds. (More on that last assumption later.)
In scenario A, you convert it now, and have $780 in your Roth. Let's say over the next 10 years, the money doubles, there are no further taxes, and you therefore have $1560 in spendable funds.
In scenario B, you wait 10 years, and your tIRA money doubles to $2,000. You then decide to spend that, and you take it out and pay your $440 in taxes, which leaves you with (drum roll...) $1560 to spend.
Mathemeticians call this "the commutative law of multiplication." There is no break-even time period.
Obviously, if the tax rate is higher later, you are better off if you convert now; and, contrariwise, if your tax rate is lower later, you would be better off waiting.
Things can get a little more complicated if you are paying for taxes from other funds, like a taxable account. This favors converting now, because you effectively move some of your money in a non-tax-preferred savings vehicle into a tax-preferred account. You can read more about that here:
https://personal.vanguard.com/pdf/ISGBETR.pdf