When should i generate the ILM?

if i want to implement 2 big cells in my block with ILM,
when can i start the top cell flow?
only after i finish routing the ilm?
or i can generate an ILM before and this way to save runtime?

thanks.

It depends on the version of ICC you are using. The latest ver of ICC which ic 2011.09 has a lot of flexibility with ILMs and lets you mix and match pre-routed ILMs. But I think that prev revisions of ICC would require you to have matching ILMs.

i.e. if you want to do just place_opt at top and stop there, you can use pre-routed ILMs for blocks as well.
But if you want to take the top through route, then your ILMs should also be from routed blocks.

My recommendation is to use routed block ILMs as they have more accurate timing and parasitics for top level integration. You may save time initially by using unrouted ILMs, but based on my exp, you will eventually end up needing routed ILMs.

Thanks,
another quesion regarding ILM's:
if i am making a change in the ilm, i should start the top flow all over again? or just load the new ILM?
because it is possible that ICC will do think differntly with the new ILM, right?

I generally don't make changes to the ILM. I would make changes to the block and then generate a new ILM. Anyways lets assume that ultimately the ILM gets changed somehow.

Starting the flow all over again ? - This depends upon how big of the change you are making. If you are making drastic changes, then its good to start all over. But for small changes, just link the new ILM and an incremental optimization will be sufficient.
One more thing to keep in mind is that it is much easier to integrate a new ILM at the pre-route stage. Once you get into routed stage, any port changes will lead to DRCs (which of course can be fixed, but is an extra step)

ICC will think differntly with the new ILM, right? - Correct. Yes, it has to think differently otherwise it will be a dumb/useless tool.