There is a variant ruleset that limits leveling to approx level7. Up to there, you can get an idea. Level 1, a house cat is considered stronger then your average commoner. A lvl 1 fighter has good endurance, and can last quite a while. The wizard can perform a number of tasks, sharply limited, while supporting with a crossbow. Other classes fall somewhere in between. Power level, a party might be able to take over a small village for a bit. By level 7, magic is seriously coming into play. A single decently built plager character could probably kill a small village, regardless of class. Any trained opponents would make it more difficult, and against an army, they'd lead the spearhead to the commander, but one or two in a party of fourbwould probably fall in route. At around level 13, a properly built mage could destroy everything. A good fighter could keep up, especially if they surprise said spellcaster. By level 20, there is no contest. Any intelligent spellcaster should be considered to never be surprised, given the amount and degree of their buffs. Fighters simply gain more feats, which are very slow and linear compared to the amount of options a mage gets.
To put it into vs army terms.
Level 1, the party are scouts, making attacks of opportunity on supplies.
Level 5, mage blasts a hole, party rushes the commander
Level 10, party stalls an army, Spartan style.
Level 15, the mage casts a single spell and the opposing army dies.
Level 20, the mage is the army, and casts a single spell, killing the entire other kingdom.