For how long can muscles rely soley on lactic acid fermentation to generate atp

For most activities it usually lasts at least 90 seconds in duration for you to solely rely with lactic acid. The ultimate energy fuel is adenosine triphosphate (ATP), which is produced in the cell. Lactic acid in the cell will itself metabolize into ATP, a process by which energy can be produced without oxygen, known as the anaerobic lactic energy system. 

Allow large amino acids through

Yes yes yes very good