Well, as with any theme park, try and go during the middle of the week and not on a holiday to avoid large crowds and long lines. I went there for the day on the Wendseday after Memormial Day, and the longest line for a roller coaster that I experienced was about 15-20 minutes for Scorpion (because they only can run one train on it). Montu, Kumba, Gwazi, and Sheikra were all less than ten minute waits when I rode them at various times throughout the day. Of course on the weekend you can expect them to be longer (maybe 20 to 45 minutes and occasionally an hour). But compared to most amusment parks, this is still pretty good.
As for Sheikra specifically, it usually has relatively short lines most of the day (except for maybe around lunch) because it is a very efficient at moving lines quickly. Generally, it is also usually a walkon in the morning, so if you want to get a few rerides on it, this is the best time to do it.
I would also recommend riding Rhino Ralley early as the lines can get fairly long later in the day. It is a fun ride, especially if you like animals, but it is not worth more than a 30 minute wait. Thats about it in terms of lines, but I would also like to advise you to take your time and enjoy all that the park has to offer. Besides rides, there are also a ton of great animal exhibits to view, and plenty of great restaurants to eat at (though this is still a themepark so the food is expensive

) Have fun!