Then the seventh angel blew his trumpet, and there were loud voices in heaven, saying, "The kingdom of the world has become the kingdom of our Lord and of his Christ, and he shall reign forever and ever." (Rev. 11:15 ESV)
If the trumpet blasts and plagues make up the great tribulation, then he would become king in the middle of it. If the tribulation is seven years, each trumpet/plague would be 180 days, depending whether these are of equal length. His kingship of the earth would only be established after he subdues the earth, 'God making his enemies a stool for his feet,' thus after Armageddon.