You can take your time by not responding to your ex but it shouldn't exceed one week. I understand your predicament since you don't want to respond negatively. It's very important you don't respond negatively because it might affect you and/or your ex negatively and you might love the rest of your life blaming yourself and might develop depression and low self-esteem.
I support your decision to need some space or time before responding. This will allow the opportunity to reflect or ponder on several things concerning the failed relationship and your ex and know what to say and not to say to your ex, and what your final decision will be. Also, you will gain the strength to be emotionally stable without reacting negatively and/or desperately when you contact him.
Send him a text acknowledging the fact he ended the relationship but you won't respond to because you need your own space to ponder about the whole thing - the relationship and your ex and you - and you will get back to him after a few days. Also, you need the courage or strength to respond to him.
You can take as many days as you want to ponder about the relationship and to gain the courage and not to become needy/desperate after you tell him that you need some space. But, it shouldn't be more than a week.
Your proposal is a good decision to undertake since you don't want to react negatively, beg/plead for a reunion or become desperate/needy to have him back.