Engine tonnage usually takes up between 40 and 60%. Fuel weight varies depending on requirements.
Armour tends to be about 3 layers deep, percentage varies depending on tech and size. Starting at 15-20k I may add shields, for about 5% of total weight.
I often aim for 10 months of deployment time and 1 year of maintenance, for varying percentages. depending on needs I may deviate far from that.
My ships tend to be very space-inefficient but either fast or cheap to operate, which extends their service life.