Somewhere between 4 hours and maybe 4 months.
Tool life depends on many factors.
How many parts per sheet?
What material are you cutting?
Some plywoods can be particularly hard on bits. Some have very abrasive glues which will where grooves in the bits at the gluelines very quickly. Some will cause resins to build up on the bit's resulting in hotter running bits, which dull quickly.
To get the maximum tool life, you need to run at the highest chiploads you can. This keeps the bits cooler. Other than abrasiveness of material, heat is the main culprit in bits getting dull.
With harder plywoods like Baltic Birch, a chipbreaker compression spiral can allow higher feedrates, which in turn can give longer tool life.
Generally, if you're cutting 4x8 sheets, with maybe 6-10 parts per sheet, expect to cut between 20-100 sheets before the bits need sharpening. Cut quality will usually tell you when to change bits. If quality isn't that important, you can use a bit longer.
You can probably get a 3/4" bit sharpened 3-4 times. Sharpening will usually run around $20/bit, and each sharpening will remove around .015" from the diameter of the bit. So after 3-4 sharpenings, you'll basically have a 5/16" bit.