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Let's do the math.
First, the initial units – 16 Imps. In the second week, another 16 Imps. Let's assume the Castle is built in the second week – from the 3rd week onwards, there are 32 Imps.
The first skirmishes begin after level 10; the larger the map, the higher this level. Let's say, on average, it's level 15. Then, without considering buildings that increase the amount of knowledge gained by heroes – the Knight, Dark Elf, and Necromancer (the weakest in terms of mana) will have 3 units of knowledge each. Usually, by this level, heroes have several artifacts, primarily addressing their weaknesses. Moreover, 1-2 knowledge points can be obtained from an artifact right from the start. Plus, 1-2 knowledge points can be gained from buildings during this time. In total, at level 15, these factions will have approximately 50 to 80 mana.
Level 15 requires 34,140 experience points. Let's assume we gain a minimum of 3,000 experience points per week (usually, we're already running around with levels 2-3 by the end of the first week, and then we fight more numerous and stronger neutrals).
So, we need 11 weeks to reach level 15: 34,140 / 3,000 = 11 weeks (or 9 weeks of full experience gain and 2 with normal gain). By this time, if we don't spend them, we'll have accumulated 16 + 16 + 32 * 9 = 320 Imps. Let's assume that 25% of the Imps are lost (usually in early battles), which is 80 Imps. Then, by level 15, we have 240 Imps. This means they will drain 240 / 8 = 30 mana from the opponent. This means that the average heroes of factions weak in mana will have between 20 and 50 mana after this.
That's pretty good! :D