How long should dancing be ?

That should be plenty of time. Do you have to be out by 10, or music done by 10? If it's out by 10 you should find out from your DJ how much time they need to break down (usually about an hour).

You should be good on your dancing time, 3-4 hours is a long time.

Speeches are usually given once everyone has food and after the bride and groom have finished eating (along with whoever is giving the speech). I would say about 30 minutes into eating. Then you do cake cutting and first dances?

A lot of DJs in my area have started to recommend doing the cake cutting right after your grand entrance since everyone's attention is already on you.

Speeches are during/after dinnertime. The time frame is personal preference. Most say they love to dance but they don't like when your reception goes over a certain "industry-standard" time, so the time allotted is very short. Unless you open the dance floor upon arrival at your cocktail hour, many wait until after-dinner festivities are done (factoring in 2 hrs for dinner). Plus any number of spotlight dances before guests can start. For most, that ends up being roughly 2 hours of dancing which is really not very long. Do what works for you and your guests.

I think it is fine. For our wedding dancing started around 7:45-8 and went until 10 when the reception ended. Speeches were given prior to dinner and then a prayer. It is completely up to you when you want to have speeches, but I'm pretty sure I've always seen them done either right before dinner or immediately following dinner.