Opening subsequent 'connections' is exponentially harder.
Using 0 donors allows the Witch to use 100% of their power. Every donor reduces the witch's available power by 5% (to maintain the conduit), but adds 40% of the donor's power. Connecting to the $N^{th}$ donor takes $3^{2N}$ seconds
From here, all calculations are made on the assumption that everyone has the same mana capacity - Witch or not.
Connecting to 1 'donor' ($Mana = 95\%^1 + 1*40\% = 135\%$) takes about 9 seconds
Connecting to the second 'donor' ($Mana = 95\%^2 + 2*40\% = 170\%$) takes another 1 minute, 35 seconds.
The third 'donor' ($Mana = 95\%^3 + 3*40\% = 197\%$) takes 12 minutes, 9 second more - total time 13 minutes, 39 seconds - and now you're about as powerful as an Evil Witch was after the first 9 seconds.
To be as powerful as a 90-second Evil Witch ($Victims = 2$ and $Mana = 95\%^2 + 2*100\% = 290\%$), would take between 5 and 6 'donors' ($Mana = 95\%^5 + 5*40\% = 277\%$ or $Mana = 95\%^6 + 6*40\% = 314\%$) and either 18½ hours or almost an entire week.
Give an "Evil Witch" that same week, and they'll be casting with $674\%$ of normal Mana. That allows for some pretty impressive spells - ones that would take you over 6 million years to match