You can start with a classic historical example: Alexander the Great. During a 10 year period (335 to 325 BC) he conquered an empire of about 2 million square miles. Since the continent of Africa covers about 11.7 million square miles (call it 12 even), this suggests a conquest time of 60 years. This is going to be a problem, since Alexander was one of those extraordinary commanders who never lost a battle, and who inspired loyalty in part by fighting with his men. Swinging a sword in battle for a 70 or 80-year-old man is not entirely realistic. Furthermore, the odds of actually surviving 60 years of battles is pretty small - Alexander was seriously injured once.
This was done with an army which varied in size in the range of 30 to 50 thousand men, about half being mercenaries. Again, a 60 year campaign is going to be a problem as the original body of troops gets whittled down. Furthermore, Alexander was stopped in India by mutiny of his troops, who hadn't been home in 8 or more years, and asking any troops to sign up for lifetime exile is unlikely to get good results.
All this means that the conquest needs to be the result of policy in a large central government, rather than the result of one man's driving ambition. The government would need to ship out both replacement and occupation troops and administrators for the conquered territories, rather like Rome, and it would need to produce both good troops and competent (rather than political) commanders for 60 years. This is, on the face of the historical record, unlikely.