In my experience you want to be as flexible and open on Fiverr as much as possible. Don’t corner yourself into a situation you can’t get out of, or find yourself on the end of a sticky situation due to an unsatisfied buyer.
As imagination said before, the 30-day limit is there for a reason. If your buyer has a large project you can offer milestones in a custom offer (I assume as a new buyer you can offer this but I might be wrong) - this means a larger project can be delivered over a period of time, e.g. $40 over 20 days so you might deliver $10 worth of work every 5 days if you want to.
I’ve had buyers offer me ‘long term’ work before, some have worked out (all I did was say I had availability for this longer term work and they just ordered via my gig every time), some have not - in that I’ve said I have time to do lots of work, and then they just stop ordering because they have internal issues, like someone leaves the company or they fall out of love with their project.
You can’t anticipate buyer’s problems, no matter how likeable they are or how much you trust them - sometimes things happen their end and you can’t do anything about it.
So, that’s why you need to protect yourself. Never take on a large project without first offering a smaller sample of work (trickier for design work, but these guys have their processes to protect their work). If you’re unsure if a buyer will back out of a large project and try to cancel an order, offer smaller milestones as I mentioned above, or a few smaller gigs first - often I’ve split orders half and half, or into 4 myself on large projects, buyers find this easier on a financial and productivity level.
Also, as you asked, don’t offer long-term deliveries unless you’ve had more than one order from the buyer and get a feel for them. Yes, they are the ones paying you, but trust me when I say some things can be such a headache that it’s not worth the order to begin with.