This should be based on one's own individual strength ... how much we have to offer from within. The voice inside us will tell us. Soulful or selfless service, looses its meaning, if we are going to burn our selves out. The secret is to always remain in a good consciousness. if you feel that you are losing this, then it is advisable to rest, re-charge and start again.
I feel that it is the same in relationships. the golden rule is that if you do not have, you can't give. All you would be doing is taking smarties from a depleted bank which will carry on being depleted.
This concept of loving oneself may at first appear selfish, but it is actually very significant in spiritual life. Pray for strength; for fortitude and above all wisdom. I hope this helps