How much weight is safe to lose per week?

I've put on about 50 lbs in the last 3 years and finally decided to get off my ass and get back into the shape I used to be in. I started in earnest approximately 6 weeks ago. In that time I've lost 18 lbs, going from 258 to 240 as of yesterday. Is 3 lbs a week a safe amount to be losing? Should I be shooting for more? Thanks in advance.

 Whatever your body is losing is good. If you're feeling "off" then perhaps you're suffering some problems in your approach. The first few weeks/months you would lose more than later on. 

If you're eating cleaner your body will naturally be adjusting. 

Occasionally losing 8 a week is okay. But what I would suggest is that for your long term health you focus on 2-4lbs a week and be okay with weeks where you lose only 1lb.

The goal is lose weight without risking a heart attack at your size. Work hard, eat clean.

I would even say focus on the intensity of the workouts, not the amount of weight lost. Because you will have periods where you will lose nothing for even a month. Focusing on your strength, cardio etc put the focus on getting healthier each week. The weight will follow on its own.

 amount of BODYFAT lost :)