Its not about the amount of metal and how long it burns. Metal burning is only the key to the power. Its not converted into it.
Purity of the metal, closeness to the Shard and also the purity of ones Allomantic Powers determine how much Power you can gain by burning. You do not go faster or slower through your metals this way. You are getting more power.
TLRs regular use of Allomantic powers was so efficient that watered down Allomancers like Vin had to use Duralumin to mimic the strength of his power. TLR was 1:1 conversion where Vin was 10:1 or something.
Like a gram of Pewter would burn for 10min for both TLR and Vin, but win would get 10x less Allomantic power for it.
