When it is time to switch to an intermediate programme?
You make quicker progress on a beginner programme so you want to stick to it for as long as you can. If you feel like you are struggling then first consider if you are eating enough? Right now you should be gaining muscle, which unless you were overweight to start with, means the scales should be going up. If they aren't then you need to go read the diet sections of this FAQ. Second, you need to consider sleep. Sleep is when most of your recovery happens, recovery is the process through which your muscles get bigger and stronger, so if you aren't sleeping enough then you will struggle. Third, consider if you really are still doing the programme. Sometimes people slowly drift off doing the programme as it is written so don't notice they have done it which goes back to the problem of you not knowing enough to be tweaking the routine. Also, make sure you have been deloading as the programme recommends.
If you have those three things in check then you can start considering when you are done with your beginner programme. Unfortunately there's no clear cut answer to this but one thing that will make it easier is if you have a written log of your training to look back on, in a note book and/or online. It means you can go back and look at your progress more objectively than say going by how you feel it's going this week. Decide to switch when you have reached a point where progress has slowed significantly and deloading no longer seems enough. It's not a clear but thing but you'll likely know it when you get there, try to push through for a bit but it doesn't really work.
Intermediate Programmes
If your goal is strength then the most common programmes to move onto after you finish with ICF, SS or SL are the Texas Method , Madcow, and 5/3/1.
By now you have been lifting a few months and will have learnt a lot so will be better placed to work out for yourself which is right for you. The articles linked to will give you a good grasp on each programme but if you are really can't decide then again just pick one. You can always change your mind later but are both good and the key thing is to be doing a good programme consistently and to stick with it for at least a few months.