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**.