Under normal amortized loans, you are always paying at least some interest and at least some principal, but there's a weird symmetry point where the total you've paid so far equals that of the loan's entire lifetime interest. It's a derived checkpoint from basic amortization math to find the month when cumulative payments so far equal the loan's total lifetime interest. In a sense, the point at which you have finished paying off your interest and then can start paying off your principal (you are, after this point, still literally paying off interest into the interest account, but it's just illustrative).
The following are under a 30yr vs a 50yr 500k loan at 6.5%
Under a 30 year loan, this occurs during month 202, around 16 years and 10 months into the loan.
(monthly rate r = 6.5%/12 ~= 0.5417%; term n = 360; payment m = p *r / [1 − (1+r)^−n] ~= $3,160.34; lifetime interest ~= 360*m − 500,000 ~= $637,722; crossing month k = n − p/m ~= 201.8 -> month 202)
Under a 50 year loan, this occurs during month 423. That's 35 years into the loan.
(monthly rate r = 6.5%/12 ~= 0.5417%; term n = 600; payment m = p * r / [1 − (1+r)^−n] ~= $2,818.58; lifetime interest ~= 600*m − 500,000 ~= $1,191,151; crossing month k = n − p/m ~= 422.6 -> month 423)
You are chaining yourself to debt not only for 20 more years, but also for nearly DOUBLE the interest. The 50-year stretches your payment schedule so far that interest compounds on interest, by the time you hit the aforementioned "interest parity" point, you've already paid more than the ENTIRE 30-year loan ever would in terms of interest.