Short answer
Use no more than 150 characters, put important information first and avoid promotional copy and gimmicky formatting.
Lead with distinguishing information
Put product type, brand, color and size early because Google may show only part of a title.
150 characters is a limit
Use space only for relevant attributes. Do not repeat words or add discounts and shipping.
Check known product data
Supply known attributes to verify their presence. Without context, a checker should not guess.
