As always in Dom2, that depends, but long sieging is most of the time not a good idea:
- I think starving out defenders is rarly the purpose of a siege: The AI's units are starving anyway without sieging and any sensible player will either lab-teleport or sneak in wine skins if relief forces are not immediately at hand...
- Furthermore, a sieging force is vulnerable for long-distance spells and specially suited counter-forces: You always know what the sieging force is composed of and where to find it. Being predictable is always bad in strategy games...
- Gate Cleavers and such even make long sieges unnecessary, which might be eventually the reason why most people choose Watch Towers anyway...
So unless castle supplies and defense points become much higher, sieging is usually not an issue. I actually like the Shepherd Scenario's Crystal Citadel, having 500 points of defense. You really have to think about taking that one, for just a bunch of SCs and Thugs holding Gate Cleavers wont rip it apart in one turn all by themselves, while each additional turn exposes you to the other human players who will gang up immediately...