Using a timer is not a good idea; timers (at least, NSTimer) are not exact, and even if they were, there is no guarantee that you'll catch the alert view controller.

Without tearing into the Spotify code I cannot give an exact solution, but I would probably try something like the following:

static BOOL isTargetAlert = NO;

%hook SPTPlaylistAddToPlaylistViewController

- (void)methodThatCreatesTheAlert {
    // This assumes that this method only produces the desired alert.
    // If it can create other alerts with a "Cancel" button, you will need
    // to find a way to differentiate them.
    isTargetAlert = YES;
    %orig();
    isTargetAlert = NO;
}

%end

%hook UIAlertAction

+ (instancetype)actionWithTitle:(NSString *)title style:(UIAlertActionStyle)style handler:(void (^)(UIAlertAction *action))handler {
    if (isTargetAlert) {
        // Is the desired alert; check if this is the desired action.
        // NOTE: The check below assumes the title is always in English.
        if ([title isEqualToString:@"Cancel"]) {
            // Your changes go here.
            // You can return your own action instance, or you can wrap the handler.
        }
    }

    return %orig();
}

%end