When I did a long 2-hour mash at 148 F, I got 79% attenuation.
With a 1-hour mash at 151 F, 74%.
And now for grins on my most recent batch, I tried the Wyeast 3787 that is supposed to be equivalent. I believe I mashed for 40 minutes at about 152 F. Went from 1.074 to 1.018, so that's 76%. Different yeast and different mash scheduled but it did indeed turn out similar.
Bottom line is, expect mid to upper 70s for most standard Belgian style recipes. All recipes above that I brewed included a small percentage (5%?) of table sugar and a very small amount (1-2%?) Special B and no significant use of crystal malts or other less fermentable malts.