If the EVSE shows its current charging power (e.g., 7 kW), then the charging time should be consistent.
If it doesn't show its charging power, then it might not be providing 7 kW consistently. If the demands on the local electrical grid are high, the voltage at the EVSE might be low which would increase the charging time. If your battery pack is quite cold or hot, your car's battery management system could be reducing the power that it is willing to accept. If your car's charging electronics (KLE or EME) aren't functioning correctly, the charging power that your car would accept could be low. The i3's original KLE was defective. BMW updated the software that controlled the KLE to reduce the charging power to protect the KLE from failure. Some replacement KLE's have failed which reduces the charging power by half.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE