Hard to predict. Ross Perot in 1992 participated in the debates and won 19% of the PV, broke into the 30s in one state (ME), but won no EV's.
Depending on the 2020 nominees, it could happen in 2020. The performance of the Libertarian party was unprecedented in 2016 at the Presidential level. If there were some sort of Libertarian-Green "Fusion" party in 2020 that perhaps combined economic moderation, prison reform (especially for nonviolent crimes), and a sensible foreign policy, it could make inroads-- though it could be another Eugene Debs (1912: 6% of PV and no EVs) or John Anderson (6.6% of PV and no EVs).
Just a guess, though.
The Libertarian Party under performed in 2016. That was really their year to shine given the unpopularity of the major party nominees and it was completely squandered.
