You could use as a baseline a potion that replaced your Conviction stat for the purposes of determining mental stress. 3 shifts gives you Good (+3) Conviction and mental stress track length of 4. 5 shifts gives you Superb (+5) Conviction, a mental stress track length of 4, and an extra mild mental consequence slot. 7 shifts gives a mental stress track length of 4 and 2 extra mild mental consequences. (So far, I believe this is standard by the RAW.)
Now, assuming you still want a mental stress track length longer than 4 rather than the extra mild mental consequences, you'd need to figure out a conversion (house rule time!). Offhand, I'd suggest one extra mild mental consequence could be cashed in for one extra mental stress box, but you could modify this exchange rate as you see fit.
Optimized Crafters can make 10-shift potions at Submerged, which in this case would be a potion granting Legendary+1 (+9) Conviction for the purposes of determining your mental stress track plus 1 shift to extend the duration of the effect. With my suggested conversion, that would be a mental stress track length of 7, boosted one step on the time chart for duration.