I'd like to thank Heart for his vote of confidence...seriously. I'm flattered.
I agree with what he says, too...ngondro is not just "grunt work" to be done before the so-called "main practice." In my case, I knew I had another practice that would take all my allotted daily practice time, and in discussion with my teacher, I agreed that I could finish the prostrations. This was carefully planned. In fact, it was clear that I could be expected to do more, later.....
"Ngondro" means "That which goes before." In one sense, it can mean "preliminary." But in another sense, it can mean that which comes before all else, or, in other words, that which is Highest Priority or Most Important. The longer one practices, I think the more one realizes the latter definition. When I have time, or am in retreat, I do full ngondro...it takes me about 25 minutes to do 100 prostrations now, plus an additional 15-20 minutes for the liturgy before and after.....
And I think it's better to take one's time, whether outside or inside of retreat, with ngondro. Five months, Six Months, in retreat. seems like an appropriate time span to really spend with these practices as one's main practice. It took me 3 years, also, to finish prostrations.....3 years to do about 80k, and 5 days to do the rest. I'm not recommending this. Especially not for someone who's now my age and weight!