Delay between requests in flatMap

I have several requests that must be called one by one:

this.firstRequest()
.flatMap(resp => {
   //check response
   if(!response.success) {
      return Observable.throw('some message');
   }

   return this.secondRequest();
})
.subscribe(...)

firstRequest() {
 // build params and call http service
 return this.service.getData(...)
   .do(r => {
      if(r.success) {
         this.localStorage.save('code', r.code)
      }
    })
 .delay(5000); 
}

As you can see there is delay between first and second request. But, I need delay only when success field is true. How can I do it?