FWIW it's about 32 miles (nautical) from the breakwater at Marina Del Rey to Bird Rock at Two Harbors. Assuming you're averaging about 5 knots, getting there should only take about 6 hours. It's the getting back that can be exciting/time consuming. It might be wise to use the search function as there have been quite a few discussions regarding visiting Catalina that can provide good information. If you have joined a Sailing Club (per some of your earlier posts), you should be able to get quite a bit of information from the management and other members as well. The trip is not especially demanding but it would be wise to develop your skills before attempting it. Most often it is pretty easy but on occassion it can be very exciting and not something for a novice.
PS: In re: your earlier post regarding worth while books, take a look at "Sailing for Dummies" by JJ and Pete Isler. A good, informative, easy to understand, read.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."