How much of a molecular cloud can end up as "starstuff"? Stars form within molecular clouds. These clouds can be up to 6 millions solar masses. When the cloud collapses into stars, is it possible to know a rough figure for how much of this material actually turns into stars? 1%? 99%?

Do larger clouds tend to be more mass-efficient in star formation than smaller ones?
This quantity is referred to as star formation efficiency (SFE) by astronomers who study star formation and galaxy evolution.   Estimates can vary but typically are around a few percent.  In Sec. 4.1 of this paper Inoue et al. review  some estimates from the literature.  Those numbers are for the central regions of spiral galaxies, and the rate may be lower in the outer regions where the density is lower.
Note that even if the rate is a few percent in a given cloud for a given episode of star formation, the rest of that gas mass will be dispersed into the interstellar medium and eventually find its way back into another cloud to form stars later - star formation is ongoing in spiral galaxies.
Note that you will often see SFE expressed as star formation rate (solar masses per year converted to stars) divided by total gas mass.  That has units of (mass/yr) / mass, which reduces to units of 1/yr.  You can think of that as the fraction of gas that forms into stars per year, or alternatively, the inverse of that number gives you a timescale in years to deplete the gas entirely.