In order o spite my girlfriend and I am going to take longer to complete the job.
That doesn't make sense. Why do you want to make it last longer?
Because if the job takes longer to finish, I'll have less time to go out with her.
it is wrong to make people wait and drag out the time. If you don't like it it might be better to break up and find a new relationship.
I really don't like her anymore.
Relationships do grow apart sometimes and it is okay to end it when it does, but doing things like making her wait so that she gets angry and ends the relationship is not a good idea. If you talk to her about the problems in your relationship at least you could end the relationship without any hard feelings.