1st a question about your coal consumption; how many pounds can it hold when full? now to answer your question about the poking from underneath, i actually have to do both ways, poke up and down and push and pull. my shaker grates are a 2 piece unit. over a month and a half ago, i broke the nub that connects the front and back grate. bummer. been like that since then. the only way i can get the ash out from the back is to, like you said, push and pull with the homemade poker between the grate and grate framework. ashes fall out beautifully. in the front i shake down 1st and then in an up and down motion get the remaining ash out so it can breathe.
depending on how hard i work the furnace, how well its drafting, and so on, i can get away with 18 hours on a single burn. done 24 hours before, but i found that i ran a 50/50 chance of having a nearly dead fire, then had to fill with wood, come back 4 hours later, stir it up, and re load. so i try not to go more than 18 hours the.
this could be partly due to my coal quality. i been burning 50% blaschak and 50% "mystery" coal that was FREE
from a basement. who knows how long its been there, ten, twenty, thirty years or more!
keep us all posted with your results and stay warm.