This is a thorny question. First, I’d like to start off by pointing out that learning any language in adulthood (as a second or third language) is a difficult task. Many researchers debate whether adult learners can ever reach the competence of native speakers. But that’s a debate for another entry. So, learning any language well–including Spanish–will require hundreds and perhaps thousands of hours of exposure to the language, opportunities to produce the language in meaningful situations, and the motivation to keep honing one’s skills. That said, the task of learning Spanish is relatively easier than learning other languages that have few similarities with English. First, the sheer number of cognates (e.g., politics = política) will facilitate the learning process. It also shares the same alphabet as English, so the task of learning a new script is not an obstacle. So in this sense learning Spanish will be easier than learning Arabic or Chinese. The experts at the Foreign Service Institute (FSI) recognize this in classifying languages into categories for the purposes of training government personnel. For example, a category I language (Spanish) will require less instructional time than a category III (Russian) or category IV (Japanese) language. At the Defense Language Institute (in Monterey, CA), those learning Spanish and those learning Japanese are expected to reach the same level of proficiency in speaking, listening, and reading–but the Spanish course is 26 weeks and the Japanese course is 64 weeks (note: these are intensive courses with 8+ hours of instruction per day).
However, being easier (in relative terms) does not mean that Spanish is easy to master. Being proficient in any language will necessarily involve knowing a large quantity of vocabulary–probably around 8000-9000 word families. This is a monumental task, even if you can rely on cognates to get you started. When in comes to grammar, Spanish has a number of obstacles for the English-speaker, including more flexible word order, grammatical gender, and highly inflected verb paradigms (each verb has 48 forms for the simple tenses alone). In addition, learning any second language requires you to make distinctions that are ignored in your native language. For example, the Spanish verbs ser and estar get translated as “to be” in English although they have very different meanings. Learning to make new distinctions in a second language, and doing it consistently, is not an easy task. Other distinctions in Spanish that cause major problems for the English speaker are aspect (preterite vs. imperfect) and mood (indicative vs. subjunctive). There are hundreds of verbs that convey differences in meaning depending on the presence of the particle ‘se’ (e.g., despedir=to fire; despedirse=to say goodbye). These are just some of the complexities of that are not easily mastered by non-native speakers, which casts doubt on the idea of Spanish being “easy.”