So, I have my MC and her love interest. One is a prince who is a fugitive of his kingdom, the other a street urchin who has a scarred past. Despite this, the MC falls in love with the love interest after much trial and tribulation. But their class difference and the MC's background prevents her from truly confessing her feelings, as she feels she's not worthy of him. Likewise, the love interest is unaware of how the MC feels, but he feels like he can't do it because he wants the MC to be her assertive self and confess. How do I make this long enough to build tension, but also not bore the audience?
The World is Not Enough:
How important is it to the story that the two EVER voice their love? Some TV shows go season after season where the romance between two characters is a low-level element of the story. I've seen books series where the MC and their love interest fall in love in the first book, but dance around it for 2-3 books, easy. Back and forth will they/won't they is a classic way to build tension and maintain interest in character development without going too deeply into something more like literary fiction. I don't think there IS an upper limit - IF the culmination of the relationship is non-critical to the continuing story, and is destined to happen eventually.
Obviously if this were a romance novel, completely different story. But in a fantasy novel, you can introduce a romance as a way to add color to the story, especially when other elements of the story need to build, but in a less compelling way. Real people have messy relations with romance, and it gives a fantasy story realistic appeal.
If there is a compelling reason for the romance to advance (the girl must love the boy so he has the mystical will to defeat true evil, their child is fated to save the world, and so forth), then that obviously takes precedent. If you want to spice up the story with some not-too-graphic fooling around, this is another reason to move it ahead.
With romance, the people can profess their love and STILL screw up the relationship if you want, moving it to the back burner because (insert reason here: rich person is pressured to not be with poor person, then later regrets their shallow decision, One is caught in an apparently compromising position, someone is betrothed in an arranged marriage, religious differences, the choice is endless). This lets you stretch the personal pain out even further. And pain is character development.
The one exception is that your reader will be angered if the story arc ends without romantic culmination. If this is a standalone novel, that means the end of the book. If a series, then by the last novel. But even then, perhaps the DENIAL of their love is the culmination - he must marry the Duchess of Doofendorf to secure the peace, or her mystical powers will disappear if she is no longer a virgin (and they both know they don't have the will to resist if together). It's a little Casablanca, but people love that stuff too.