Renal threshold for glucose
Classic symptoms of untreated diabetes include excessive thirst and urination, and glycosuria. Think about the solute gradient. At some arbitrary number the level of glucose becomes saturated. The kidneys are forced to get rid of the extra glucose in urine. This increases the concentration of solutes in the urine, so where does the water go? Right? So that high blood glucose concentration translates into frequent trips to the bathroom, and frequent need to consume fluids. This can also lead to a fluid/electrolyte imbalance, which is potentially dangerous.

There's actually a limit to what can be reabsorbed, denoted the transport maximum,
but, you can only eliminate so much solute over time, so here's a figure from a Pearson physiology chapter:



Source

You've got your renal threshold around that 180 mg/dL but look at that excretion kinetic. It's known that the excretion rate increases linearly with the filtration rate above the renal threshold (ref). So say i'm an average male with 400 mg/dL plasma glucose and 50 dL of blood or 20 g of net plasma glucose. All other things held equal, it looks like it could take around 2 hours to clear that glucose on excretion alone, which doesn't typically happen. Clearance is also outlined in that Pearson chapter, and you can measure that by knowing the excretion rate, and the concentration of glucose in the urine and in the plasma.

Data shows, however, that the filtration/reabsorbtion process is maladapted in a diabetes model, and increased plasma glucose levels are maintained by an up-regulation in glucose transporters (thus an increase in the transport maximum), and an increase in the renal threshold (and thus the kidneys only begin to excrete glucose at a significantly higher level) (ref).