No, there is no way to calculate time required for rebuilding indexes.
If the table has gone through a lot of DELETEs, rebuilding indexes will create some free space. But, its only temporary. If the table keeps on undergoing similar DML activity, the index will grow again.
Well, you could rebuild indexes and coalese free space, but what happens if that index you have rebuilt would have expanded into the whitespace that you have just "shrunk"? Lets say you have an index like this with leaf blocks a bit like this:
It is rebuilt to look like this:
Then you enter NOTTAGE and ANDREWS into the table that the index references, not only you have just expanded your index back to where it was (using the same amount of space), but you have introduced the un-nessessary overhead of widening the index again.
In other words, unless you are sure you need to rebuild an index (each index has been analyzed on its own merits), don't worry about it just:
BUY MORE DISK (and allocate it to the tablespace where you hold your indexes)